Just use flush_frontbuffer directly.  The flush_frontbuffer routine has
been somewhat devalued recently, but it is actually just the right
interface for our needs.

It is in pipe_screen, meaning that any wrapping (eg trace module)
will get properly unwrapped before we try and use the pipe_surface
argument for real.

If a particular co-state-tracker needs to implement this itself, it
should organize a way to allow the winsys to call back up to its
level, rather than hijacking the driver-supplied implementation.