Added mipmapping, trilinear and anisotropic filtering.
- ManagedGLContext now can have a filtering set to control how the textures are set up in its context
- the filtering can be set globally for the game in setting.xml
- textures now generate mipmaps and can be set not to as well
- textures are loaded using the TextureDescriptor class that can load <texture> tags
- <texture> tags now can have an additional attribute to disable mipmapping for that texture