Whenever the source area of the texture being copied is out of
bounds and robust resource init is enabled, it is necessary to
clear the invalid portion of the source area to 0 in the copy
operation, EXCEPT if the source and the destination are the same,
in which case clearing the destination would also clear the source.

Bug: angleproject:3930
Change-Id: Ie9e0c52fff86b5626c82a511319688392b7b073b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2391281Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>