monitor-stream-src: Use cogl_framebuffer_finish()

Even though cogl_framebuffer_flush() was supposed to be enough,
it ends up creating streams with odd visual glitches that look
very much like unfinished frames.

Switch back to cogl_framebuffer_finish(), which is admittedly
an overkill, but it's what works for now. There is anedoctal
evidence showing it doesn't incur in worse performance.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/1086

src/backends/meta-screen-cast-monitor-stream-src.c

@@ -436,7 +436,7 @@ meta_screen_cast_monitor_stream_src_blit_to_framebuffer (MetaScreenCastStreamSrc
         }
     }
 
-  cogl_framebuffer_flush (framebuffer);
+  cogl_framebuffer_finish (framebuffer);
 
   return TRUE;
 }