The frontend format is not always valid to use for glCopyTexImage.
Translate it to a valid format for the current driver.

TEST=deqp/functional/gles3/framebufferblit/default_framebuffer_00.html

BUG=angleproject:4632

Change-Id: I838a1b27b528f074f440575adb88357a910f596a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2196842Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>