The copySubImage LUMA workaround sets special TEXTURE_SWIZZLE_RGBA
values to work. However, it must reset this parameter back to the
default state after the workaround completes.

Bug: 1022080
Change-Id: I84870f1264311c106ebece1e6a37d5f92a1da28c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1904627Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>