Use ImageHelper staging buffers for copyImageDataToBuffer 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/+/2425250 Reviewed-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>