Revert change from using context staging buffers for
copyImageDataToBuffer. There are scenarios where the
staging buffer will become invalid before flushing
the staged update. Added a test for this case.

Bug: angleproject:5092
Test: angle_end2end_tests --gtest_filter=*ETC1CompressedImageDraws*
Change-Id: I41c457fda919938600c20336f65836952d73748a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2425250Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>