CopyTexSubImage does not have the format combination restriction
that CopyTexImage has. The destination may have components not
present in the source image.

Bug 21610276

Change-Id: I90f4c0679ceb1061b05a404aa6ca817205b5077f
Reviewed-on: https://swiftshader-review.googlesource.com/3361Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>