culling: Don't cull actors not being painted on the stage

Previously we were applying the culling optimization to any actor
painted without considering that we may be painting to an offscreen
framebuffer where the stage clip isn't applicable.

For now we simply expose a getter for the current draw framebuffer
and we can assume that a return value of NULL corresponds to the
stage.

Note: This will need to be updated as stages start to be backed by real
CoglFramebuffer objects and so we won't get NULL in those cases.

cogl/cogl.h

@@ -1282,6 +1282,9 @@ _cogl_onscreen_clutter_backend_set_size (int width, int height);
 void
 _cogl_swap_buffers_notify (void);
 
+CoglFramebuffer *
+_cogl_get_draw_buffer (void);
+
 G_END_DECLS
 
 #undef __COGL_H_INSIDE__