window-stream-src: Finish framebuffer after blitting

Just like what's done for monitor screencasting. Unfortunately, there's
no mechanism to share fences with PipeWire clients yet, which forces
us to guarantee that a frame is completed after blitting.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/1129

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

@@ -492,6 +492,8 @@ meta_screen_cast_window_stream_src_blit_to_framebuffer (MetaScreenCastStreamSrc
       break;
     }
 
+  cogl_framebuffer_finish (framebuffer);
+
   return TRUE;
 }