The issue was fixed, but adding a regression test to avoid it
again. glMemoryBarrier inserts a memory barrier and breaks the
submission order, which causes the copyBufferToImage happens
before copyImageToBuffer and image corruption. This test fails
on commit 052167bc, and can pass on the latest build.

Bug: angleproject:4074
Change-Id: Icc42dd7b2ba167815b223085ce933682654ce760
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1943848
Commit-Queue: Ian Elliott <ianelliott@google.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>