bug=angle:582

Also performs format checks for non-subimage copies, and permits unsized format
textures as destination for the copy.

Change-Id: Id647bb968416fbd35e1c70e5da21c81367e08f07
Reviewed-on: https://chromium-review.googlesource.com/189431Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>