Enable adjustSrcDstRegionBlitFramebuffer workaround on iOS and fix
a conflict when it is enabled in combination with
clipSrcRegionBlitFramebuffer.

Bug: angleproject:5417
Change-Id: I1de0f9f392643fbcf6900bad86d83084cb6d4009
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2601106Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>