Draw now has its own staging buffer to use with its scrap. Also, a few
fixes were needed for the staging buffer and scrap flush routines.
Other than some synchronization issues with draw scrap flushing
(currently worked around with a fence-wait) things seem to be working
nicely.
The scrap texture did very good things for the glsl renderer and the
better control over data copying might help it do even better things for
vulkan, especially with lots of little icons.
