actor: add oob-transform opt to catch out-of-band transforms

Out-of-band transforms are considered to be all actor transforms done
directly with the Cogl API instead of via ClutterActor::apply_transform.

By running with CLUTTER_DEBUG=oob-transform then Clutter will explicitly
try to detect when un-expected transforms have been applied to the
modelview matrix stack.

Out-of-band transforms can lead to awkward bugs in Clutter applications
because Clutter itself doesn't know about them and this can disrupt
Clutter's input handling and calculations of actor paint-volumes
which can lead to visual artifacts.

Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>

clutter/clutter-main.c

@@ -167,7 +167,8 @@ static const GDebugKey clutter_debug_keys[] = {
   { "multistage", CLUTTER_DEBUG_MULTISTAGE },
   { "animation", CLUTTER_DEBUG_ANIMATION },
   { "layout", CLUTTER_DEBUG_LAYOUT },
-  { "clipping", CLUTTER_DEBUG_CLIPPING }
+  { "clipping", CLUTTER_DEBUG_CLIPPING },
+  { "oob-transforms", CLUTTER_DEBUG_OOB_TRANSFORMS }
 };
 #endif /* CLUTTER_ENABLE_DEBUG */