resample: Additional options for constant animation channels

[donmccurdy]

As discussed in KhronosGroup/glTF-Sample-Models#86 (comment), the current behavior of glTF Transform is to reduce constant animation channels down to two keyframes. The start and end keyframes are kept because they might have semantic meaning, but in most cases they probably don't, and glTF Transform should provide options to do more:

• (A) reduce to 1 keyframe (start only)
• (B) remove constant animation channels entirely

Note that constant channels should not be removed if they differ from the rest pose.

[kzhsw]

Also have a look at #926