Significant refactoring related to graphics

GameState:
- GameState now looks for loadingLoop and loadingScreen

Graphics:
- Added hexToARGB function, which converts 6-digit RGB or 8-digit ARGB
hex values to percentage arrays
- makeCompositeTexture, in the absence of a path, will compose one
using two config constants and a name value
- makeCompositeTexture will use hexToARGB when passed a string for color
- EntitySprite is now an object extending Draw2DSprite
- EntitySprite is now constructed with a reference to its parent Entity

Entity:
- Newly constructed entities will register a TextureInstance with the
TextureManager
- Added functions to get, set, and composite to the TextureInstance