Also adds a small bugfix in
PixelBuffer::stageSubresourceUpdateFromFramebuffer where we weren't
using the right format for the pack pixels params. The format should
be unsized so that the fastcopyfunc / colorwritefunc / colorreadfunc
are found correctly if needed when packing pixels.

Bug: angleproject:2653
Bug: angleproject:2501
Change-Id: Ie4aacc623ddc9583360ab0c99be19d3db2bdbf44
Reviewed-on: https://chromium-review.googlesource.com/1104395
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>