offscreen-effect: Add private API for getting the target size

Since the FBO target might have a different size than the mere paint box
of the actor, we need API to get it out of the ClutterOffscreenEffect
private data structure and on to sub-classes.

Since we cannot add new API in a stable cycle, we need a private
function; we'll leave it there even when opening 1.7, since it's useful
for internal purposes.

clutter/clutter-offscreen-effect-private.h

@@ -0,0 +1,14 @@
+#ifndef __CLUTTER_OFFSCREEN_EFFECT_PRIVATE_H__
+#define __CLUTTER_OFFSCREEN_EFFECT_PRIVATE_H__
+
+#include <clutter/clutter-offscreen-effect.h>
+
+G_BEGIN_DECLS
+
+gboolean        _clutter_offscreen_effect_get_target_size       (ClutterOffscreenEffect *effect,
+                                                                 gfloat                 *width,
+                                                                 gfloat                 *height);
+
+G_END_DECLS
+
+#endif /* __CLUTTER_OFFSCREEN_EFFECT_PRIVATE_H__ */