Add subtextures

[ikeycode]

We need the ability to split a texture into child textures with their own UV coordinates.
This is required so we don't keep reloading textures for the sake of a child node and will
greatly simplify sprite animations.

We need to be able to evenly split by rows, columns, returning a set of textures.
Additionally we need to be able to split by specific regions. Eventually we want a
texture cache which can sort by the parent handle to prevent unnecessary
texture switches for child textures (such as spritesheets)