Implemented API level of glFramebufferTextureLayer.
All functions should now be adapted to handle level
and layer, all the way to RenderbufferTexture3D,
where layer is still unimplemented.

Change-Id: Id5fe94f998ee517ae84cb0d6e60d535cc7891fe3
Reviewed-on: https://swiftshader-review.googlesource.com/3320Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>