Write docs for new Layer system

[profexorgeek]

I need to document what Layers are and how to use them. Raw notes from chat w/ @KallDrexx:

Layer

• Composite sprite.
• Drawn once to a buffer.
• Can be "scrolled" which will wrap pixels in X and Y dimensions.
• Can track a camera offset for when camera is implemented
  • Don't really understand this yet
• Layers can support transparency but are faster if it doesn't.
• Layers have their own transparency color setting
  • TransparentColor: the color to consider transparent when drawing sprites TO this layer
  • BackgroundColor: the color to fill the layer background with. This should be the same as the transparent color if you want transparency
• Call "Shift" on layers to shift by a certain velocity to get parallax scrolling effect. This is usually done in the tick/update/activity method
• Sprite rendering happens over the top of layers just like simple rendering

LayerManager

• Add layers to the LayerManager to have them show up in the screen
• Cannot set layer index to 0 because this is the sprite layer
• Sprites can only be drawn at layer index 0 and be sandwiched betweeen layers
• Layer manager controls rendering.
• Currently must unload layers when unloading screen

TextureManager

• Split out loading/unloading and dealing with texture manager
• TextureManager was created to make this code more reusable
• May eventually become a broader ContentManager

[profexorgeek]

@KallDrexx I added you to this in case you get to it before me. I've been writing docs in the /docs folder at root as just simple markdown