Use BlitGL to reimplement copyTex[Sub]Image2D on iOS.

Bug: angleproject:4674
Change-Id: Ie3018d6d33da57797162922410f76557124df4b6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2222718
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>