Update ValidateCopyImageSubDataBase() to take into consideration if the
entire mip level is being written, even if the size of the mip level is
smaller than the compressed block size.

This also uncovered a bug in ImageHelper::initializeNonZeroMemory()
where the image extents are not at least as large as the compressed
format block size:
    VUID-vkCmdCopyBufferToImage-imageExtent-00207

This CL adds isAllocateNonZeroMemoryEnabled() to allow skipping the test
when that feature is enabled while that bug is chased.

Bug: angleproject:5592
Bug: angleproject:5634
Bug: angleproject:5643
Test: Texture2DTestES3.CopyCompressedImageMipMaps
Change-Id: I0381e0d3490fb148604b61dc3bae8f96ba8b5f8e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2673069
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>