Rename math/helpers to math/math_helpers

* Rename math test to MathTest.
Remove deprecated SpriteBatch, AnimatedSprite ctors

The ctors have been replaced with ::set_sprite_sheet.

* SpriteSheet: Rename ::load to ::load_file.

* AnimatedSprite: Fix internal updating; this class can be used
without an explicit call to ::play now, when used in like a
regular nom::SpriteBatch.

* Sprite: Remove state class var, Sprite(int,int) ctor.
* Sprite: ::load has been replaced with ::set_texture; this has been
done in order to ease sharing && decouple loading of resources.
* Sprite: Remove scale_factor class var; nom::Texture takes care of
this now.

* Texture: Change return type of ::clone to Texture* && rename
::update to ::update_pixels.
* Texture: Introduce ::scale_factor (getter); a scale factor is now
set internally within ::resize.

* Update applicable examples && tests.
* Add sprites example (based off app example, without GUI).