For textures that are backed by external memory,
immediately flush sub image updates instead of
staging them.

Bug: angleproject:4828
Tests: angle_end2end_tests --gtest_filter=ImageTest.UpdatedExternalTexture*
Change-Id: I51e5bd0cb5df7df3af21f0cdb3007eebc1be29cd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2290490Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>