A couple cases are left unimplemented:

Incomplete/unused Textures. This leads to a slightly more tricky
implementation. Since we need to read back from the staging buffer
we will need to flush the Image contents to a temporary buffer.

Depth/stencil readback. Requires a more complex pixel packing
step.

3D/Cube/2D Array readback. Also requires a more complex packing
step.

Bug: angleproject:3944
Change-Id: Ic5d9a606177ba7e3e5ab945feb5f555afa11741f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1879964
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>