This change implements staging image/texture copies when the destination
image is not yet fully initialized.  With this change, CPU readback for
glCopyTex[Sub]Image2D and glCopy[Sub]TextureCHROMIUM should happen only
if the texture formats don't allow a fragment-shader based copy.

Bug: angleproject:2958
Change-Id: I04087e14ea8fb6fbc731598c5493e44651c22c01
Reviewed-on: https://chromium-review.googlesource.com/c/1393909
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>