I had missed that vkCmdCopyImage requires the source and destination
images to have exactly the same size, and I guess assumed that the
swapchain images would always be the size they said they were, but this
is not the case for tiled-optimal images. However,
vkCmdCopyImageToBuffer does the right thing regardless of the source
image size.
This fixes the skewed screenshots when the window size is not a multiple
of 8 (for me, might differ for others).
I think I had gotten lucky with captures not being corrupt due to them
being much bigger than all but the L3 cache (and then they're over 1/2
the size), so the memory was being automatically invalidated by other
activity. Don't want to trust such luck, though.
