Support glFramebufferTextureLayer by correctly handling layers
from 3D and 2DArray textures.  Modeled after CubeMap layers support.

Bug: angleproject:3188
Bug: angleproject:3189
Change-Id: Ic73a6017134e9d2b49beed103487454397a97167
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1738436Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>