When copying to a non-cube-complete level of a texture we would fail
because we would try to get the base level's current format. Instead
we can get the format of the level that is being copied to.

Bug: angleproject:2911
Change-Id: Ib6a5c5cef67ad9c880c1b76ea71ed9317eb20c97
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1483951
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>