Copying data from the wrong buffer was the cause of the corrupted brush
model vertices, and then lots of little errors (mostly forgetting to
multiply by bpp) for textures.
I had originally planned on mixing the stage management with general
texture support code like I did in glsl, but I think that was a mistake
and I did keep looking for scrap.[ch] when I wanted to edit something to
do with the scrap...
There's still a problem with the vertex data itself not getting sent to
the GPU properly, but vulkan is now happy with my tiny test map (which
required disabling skies entirely until I get null textures working).
