This gets screenshots working again. As the implementation is now a
(trivial) state machine, the pause when grabbing a screenshot is
significantly reduced (it can be reduced even further by doing the png
compression in a separate thread).
It turns out that using the swapchain image for the size requirements is
unreliable: when running under renderdoc, vkGetImageMemoryRequirements
sets the memory requirements fields to 0, leading eventually to a null
memory object being passed to vkMapMemory, which does not end well.
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).
Id Software had pretty much nothing to do with the vulkan renderer (they
still get credit for code that's heavily based on the original quake
code, of course).
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.
