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.

Bug: angleproject:3930
Change-Id: If1605b917097006c37e7d59c66babbc32d0ba6d8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2378004Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: back sept 10 - Jamie Madill <jmadill@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>