From Tom Jolley, Added a new enum POINT_ROT_WORLD_Z_AXIS and a new section in computeMatrix for this enum.

With a small tweak from Robert Osfield to set _cacheMode.

include/osg/Billboard

@@ -112,6 +112,7 @@ class SG_EXPORT Billboard : public Geode
             AXIAL_ROT_X_AXIS=AXIAL_ROT+1,
             AXIAL_ROT_Y_AXIS,
             AXIAL_ROT_Z_AXIS,
+            POINT_ROT_WORLD_Z_AXIS,
             CACHE_DIRTY
         };