Added isImageTransulacent() method.

include/osg/Image

@@ -200,6 +200,9 @@ class SG_EXPORT Image : public Object
         
         /** converts a single image into mip mapped version image.*/
         void computeMipMaps();
+        
+        /** return true of this image is translucent - i.e. it has alpha values that are less 1.0 (when normalized).*/        
+        bool isImageTranslucent() const;
 
     protected :