framebuffer: Adds experimental _finish() API

This adds a new experimental function, cogl_framebuffer_finish(), which
can be used to explicitly synchronize the CPU with the GPU. It's rare
that this level of explicit synchronization is desirable but for example
it can be useful during performance analysys to make sure measurements
reflect the working time of the GPU not just the time to queue commands.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

cogl/cogl-framebuffer.c

@@ -2018,6 +2018,13 @@ cogl_framebuffer_remove_swap_buffers_callback (CoglFramebuffer *framebuffer,
   winsys->onscreen_remove_swap_buffers_callback (onscreen, id);
 }
 
+void
+cogl_framebuffer_finish (CoglFramebuffer *framebuffer)
+{
+  _cogl_framebuffer_flush_journal (framebuffer);
+  GE (framebuffer->context, glFinish ());
+}
+
 void
 cogl_onscreen_set_swap_throttled (CoglOnscreen *onscreen,
                                   gboolean throttled)