mesa/st: Reduce the number of frontbuffer flush calls

mesa/st: Reduce the number of frontbuffer flush calls

The mesa state tracker was needlessly flushing the front buffer even if it
hadn't been drawn to since the last flush. This was happening during
glXSwapBuffers if we at some point previously had set that frontbuffer as
a read- or draw renderbuffer, or at glFlush() or glFinish() if we at some
point previously had rendered to the front buffer. Since the frontbuffer
flush typically means a full drawable copy, it's a pretty big waste.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>