fixes for bugs submitted by BGE users, fixes by myself and Mitchell Stokes

- when the attribute check function failed it didnt set an error raising a SystemError instead
- Rasterizer.getMaterialMode would never return KX_BLENDER_MULTITEX_MATERIAL
- PropertySensor value attribute checking function was always returning a fail.
- Vertex Self Shadow python script didnt update for meshes with modifiers.

source/gameengine/GameLogic/SCA_PropertySensor.cpp

@@ -295,9 +295,8 @@ CValue* SCA_PropertySensor::FindIdentifier(const STR_String& identifiername)
 
 int SCA_PropertySensor::validValueForProperty(void *self, const PyAttributeDef*)
 {
-	bool result = true;
 	/*  There is no type checking at this moment, unfortunately...           */
-	return result;
+	return 0;
 }
 
 /* ------------------------------------------------------------------------- */