Follow OpenGL terminology for texture binding units.

Until now the textures were bound to layers, which was rather confusing,
especially when binding layered textures to layers (gaah). Also the
wording might have implied that each texture must be in some layer in
order to make it usable in shader. This is no longer the case with (yet
unimplemented) bindless texture, so another reason to remove the
confusion.

All occurences of texture layers were replaced texture binding units to
follow OpenGL naming. It was mostly in the docs, except for
already-deprecated *Layer enums in shaders, but they will be removed
soon anyway.
Doc++