Fix updates to play state definition.

See issue #196.

Commit 9e2053f updated the definition
of the play state but it failed to check for pending tasks. As a result,
when we are play-pending, we'd report 'paused' since we don't have
a resolved start time.

I've gone through all the possible states and revised this based on the
following chart. Hopefully it's right this time.

+---------+-------+------+-------+--------------------------------------+
| Current | Start | Play | Pause | State / Notes                        |
|  time   |  time | task |  task |                                      |
+---------+-------+------+-------+--------------------------------------+
|    o    |   o   |  o   |   o   | 'idle'                               |
+---------+-------+------+-------+--------------------------------------+
|    o    |   o   |  o   |   x   | [1] Normally pausing sets the hold   |
|         |       |      |       | time (current time) but if the       |
|         |       |      |       | timeline is cleared while pending    |
|         |       |      |       | this could happen.                   |
|         |       |      |       | We should report 'paused' because    |
|         |       |      |       | we won't be ready until we have a    |
|         |       |      |       | current time to use and at that      |
|         |       |      |       | point we will end up being in the    |
|         |       |      |       | regular paused state: resolved       |
|         |       |      |       | current time, unresolved start time. |
+---------+-------+------+-------+--------------------------------------+
|    o    |   o   |  x   |   o   | [2] Not expected to happen because   |
|         |       |      |       | we don't expect to play without      |
|         |       |      |       | either a start time or hold time     |
|         |       |      |       | (unless we had an aborted pause and  |
|         |       |      |       | the timelineline was cleared?).      |
|         |       |      |       | If this were to happen then          |
|         |       |      |       | presumably we believe we can run the |
|         |       |      |       | pending play task (or else we would  |
|         |       |      |       | have canceled it) so we should       |
|         |       |      |       | 'running' or 'finished'.             |
+---------+-------+------+-------+--------------------------------------+
|    o    |   o   |  x   |   x   | [3] Cannot have two pending tasks at |
|         |       |      |       | once.                                |
+---------+-------+------+-------+--------------------------------------+
|    o    |   x   |  o   |   o   | 'idle'                               |
+---------+-------+------+-------+--------------------------------------+
|    o    |   x   |  o   |   x   | As with [1], 'paused'.               |
+---------+-------+------+-------+--------------------------------------+
|    o    |   x   |  x   |   o   | As with [2], 'running' / 'finished'. |
+---------+-------+------+-------+--------------------------------------+
|    o    |   x   |  x   |   x   | As with [3], n/a.                    |
+---------+-------+------+-------+--------------------------------------+
|    x    |   o   |  o   |   o   | 'paused'                             |
+---------+-------+------+-------+--------------------------------------+
|    x    |   o   |  o   |   x   | 'paused'                             |
+---------+-------+------+-------+--------------------------------------+
|    x    |   o   |  x   |   o   | 'running' / 'finished'               |
+---------+-------+------+-------+--------------------------------------+
|    x    |   o   |  x   |   x   | As with [3], n/a.                    |
+---------+-------+------+-------+--------------------------------------+
|    x    |   x   |  o   |   o   | 'running' / 'finished'               |
+---------+-------+------+-------+--------------------------------------+
|    x    |   x   |  o   |   x   | 'paused'                             |
+---------+-------+------+-------+--------------------------------------+
|    x    |   x   |  x   |   o   | 'running' / 'finished'               |
+---------+-------+------+-------+--------------------------------------+
|    x    |   x   |  x   |   x   | As with [3], n/a.                    |
+---------+-------+------+-------+--------------------------------------+

Author: birtles

Overview.bs

@@ -1711,7 +1711,8 @@ non-normative description is also provided:
 <div class=informative-bg>
 
 :   <a lt="idle play state">idle</a>
-::  The <a>current time</a> of the animation is <a>unresolved</a>.
+::  The <a>current time</a> of the animation is <a>unresolved</a> and
+    there are no pending tasks.
     In this state the animation has no effect.
 :   <a lt="running play state">running</a>
 ::  The animation has a resolved <a>current time</a> that changes on each
@@ -1731,17 +1732,24 @@ condition from the following:
 
 <div class="switch">
 
-:   The <a>current time</a> of <var>animation</var> is
-    <a>unresolved</a>,
+:   <em>All</em> of the following conditions are true:
+    *   The <a>current time</a> of <var>animation</var> is <a>unresolved</a>,
+        <em>and</em>
+    *   <var>animation</var> does <em>not</em> have <em>either</em>
+        a <a>pending play task</a> <em>or</em> a <a>pending pause task</a>,
 ::  &rarr; <dfn lt="idle play state">idle</dfn>
-:   The <a lt="animation start time">start time</a> of <var>animation</var> is
-    <a>unresolved</a>,
+:   <em>Either</em> of the following conditions are true:
+    *   <var>animation</var> has a <a>pending pause task</a>, <em>or</em>
+    *   <em>both</em> the <a lt="animation start time">start time</a> of
+        <var>animation</var> is <a>unresolved</a> <em>and</em> it does
+        <em>not</em> have a <a>pending play task</a>,
 ::  &rarr; <dfn lt="paused play state">paused</dfn>
-:   For <var>animation</var>,
-    <a>animation playback rate</a> &gt; 0 and
-    <a>current time</a> &ge; <a>target effect end</a>; or<br>
-    <a>animation playback rate</a> &lt; 0 and
-    <a>current time</a> &le; 0,
+:   For <var>animation</var>, <a>current time</a> is <a>resolved</a> and
+    <em>either</em> of the following conditions are true:
+    *   <a>animation playback rate</a> &gt; 0 and
+        <a>current time</a> &ge; <a>target effect end</a>; <em>or</em>
+    *   <a>animation playback rate</a> &lt; 0 and
+        <a>current time</a> &le; 0,
 ::  &rarr; <dfn lt="finished play state">finished</dfn>
 :   Otherwise,
 ::  &rarr; <dfn lt="running play state">running</dfn>