mesa/st: Fix frontbuffer rendering regression

This fixes a regression introduced with commit
"mesa/st: Reduce the number of frontbuffer flush calls"
where we, after flushing the front buffer marked it as not-rendered-to,
the idea being that it should be marked as "rendered-to" again as soon as
any rendering was touching the front.

Now the latter part never happened, because it was part of a state
validation and we never marked that part of the state as dirty.

So mark the framebuffer state dirty after a frontbuffer flush.
(fdo bugzilla 102496)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102496
Fixes: eceb671002 (mesa/st: Reduce the number of frontbuffer flush calls)
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Tested-by: Bruce Cherniak <bruce.cherniak@intel.com>
Tested-By: Gert Wollny <gw.fossdev@gmail.com>

src/mesa/state_tracker/st_manager.c

@@ -1044,6 +1044,9 @@ st_manager_flush_frontbuffer(struct st_context *st)
       stfb->iface->flush_front(&st->iface, stfb->iface,
                                ST_ATTACHMENT_FRONT_LEFT);
       strb->defined = GL_FALSE;
+
+      /* Trigger an update of strb->defined on next draw */
+      st->dirty |= ST_NEW_FB_STATE;
    }
 }