This fixes issue "3" from here: https://github.com/yquake2/ref_vk/issues/3
This differs from GL3 in that it still supports vk_picmip.
Vk_Upload32 is no longer used, but I left it in case it has some use in the future.
This was first developed in a feature branch in the main yquake2 repo.
It was merged into master in early 2021, but the experiences of the
following month showed that it is not ready for prime time. There're
glitches with 3rd party assets, restarts are still shaky, etc. Having
the code in a separate repo allows us to:
* Release Vulkan independent if YQ2.
* Give commit access to contributors interested in Vulkan.
This code is the same as in yquake/yquake2 ecdf912713eef55d6c5d5a772259b44e3fc232c4.
