Support RGB -> BGRX copies in copySubImage. This was showing up in the following test for me on OSX: IOSurfaceClientBufferTest.CopySubImageToBGRX8888IOSurface/ES2_Vulkan_SwiftShader Test: angle_end2end_tests on OSX 10.15 Bug: angleproject:5147 Change-Id: I666dc832e8be33cde1617c849990c6092f4e2240 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2462041 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>