Revamped the texture system in Nausea.

Textures now cannot be directly transfered to Entity's material.
Instead, there's a Sprite class that deals with this tasks. Sprite is created with a reference to texture. Sprite can also be mapped to only a part of the texture (spritesheet).
This commit also introduced new functionality to make sure that working with Sprites won't be more of a hassle then before.
Use object's Texture.CreateSprite() to quickly create a sprite from texture.