Create a BillboardList subclass of SpriteList.

[DragonMoffon]

A simple subclass for advanced users who are using the new cameras outside the standard 2D plane.

Depends on PR #2047

[pushfoo]

If this new billboard class isn't going in experimental, it's a good idea to add:

1. Under doc/example_code/how_to_examples:
   1. A billboardlist_isometric.png screenshot file
   2. A billboardlist_isometric.rst file modelled off the others which includes:
      1. an :orphan: directive at the top
      2. a .. _billboardlist_isometric: link target
      3. A title using = to underline it / mark it as a heading
      4. An .. image:: billboardlist_isometric.png directive to include the screenshot as in other examples
      5. A brief explanation if needed
      6. A .. literalinclude:: ../../../arcade/examples/billboard_isometric.py directive like the other files to include the py source
2. A cross-links to the example as included in the suggestion comments on this PR further down

[pushfoo]

I'd clean up the intro section as follows.

Suggested change

	"""
	The purpose of the BillboardList is to batch draw a list of sprites.
	The difference between the BillboardList and a basic SpriteList is how it
	orientates Sprites. The normal SpriteList always aligns them to the X-Y plane.
	The BillboardList purposely forces the sprite to always look directly towards
	the screen.
	"""Draw sprites to always face the screen regardless of camera orientation.
	Since the basic :py:class:`~arcade.SpriteList` draws every :py:class:`~arcade.Sprite`
	as aligned with the X and Y axes, it allows perspective and camera position to affect
	how sprite polygons are drawn. This class differs by using a special default shader
	program to always make sure the sprite polygons face the camera. This is also known
	as billboarding or billboard-style sprites.

[pushfoo]

Suggested change

	For the advanced options check the advanced section in the
	arcade documentation.
	To learn more, please see:
	* The :ref:`spritelist_billboardlist` example
	* The :py:class:`~arcade.sprite_list.SpatialHash` class
	* The notes on :ref:`collision_detection_performance`

[pushfoo]

I'd rename this something like billboardlist_isometric.py so it's shorter to type when users type python -m arcade.examples.billboardlist_isometric

[pushfoo]

Remember to update this line to match any top-level file rename.

Suggested change

	python -m arcade.examples.isometric_with_billboards
	python -m arcade.examples.billboardlist_isometric