Begins transform feedback when rendering with an xfb-enabled
pipeline bound, and ends transform feedback as needed, while
writing back the counters supplied by the app. This does not
yet support transform feedback queries or the draw command.
Instead of inserting a barrier after every single buffer copy, update
or clear operation, we batch them up and execute the barrier when the
first dirty buffer is used by a command. This significantly reduces
the number of pipeline barriers in some games, e.g. Final Fantasy XV.
This is the first step to optimizing buffer updates for applications
that frequently invalidate buffers. The goal is to reduce the number
of buffer allocations per frame and reduce the cost of invalidation.
