st/mesa: Make the frontbuffer visible on st_flush(PIPE_FLUSH_FRAME).

st/mesa: Make the frontbuffer visible on st_flush(PIPE_FLUSH_FRAME).

So far the frontbuffer was only being flushed on st_glFlush and
st_glFinish, however, a co-state tracker may need to make sure that
any frontbuffer changes are already on its way to the actual front.

The dri2 state tracker will need this for event-driven GL applications
to resize properly (It could also be done calling "dri_flush_frontbuffer",
but that way we would flush unnecessarily in the double-buffered case).

Additionally this patch avoids flushing the mesa rendering cache if
PIPE_FLUSH_RENDER_CACHE wasn't specified.