evas vg: add warning msgs for users.

Since usage has been changed under the beta version,
Some users may keep the old-usage that breaks the vg behavior now.

For their information, vg prints messages in case of breaks.

src/lib/evas/canvas/evas_vg_container.c

@@ -189,6 +189,9 @@ _efl_canvas_vg_container_efl_duplicate_duplicate(const Eo *obj,
 EAPI Efl_VG*
 evas_vg_container_add(Efl_VG *parent)
 {
+   /* Warn it because the usage has been changed.
+      We can remove this message after v1.21. */
+   if (!parent) CRI("Efl_VG Container doesn't allow null parent!");
    return efl_add(MY_CLASS, parent);
 }
 

src/lib/evas/canvas/evas_vg_shape.c

@@ -426,6 +426,9 @@ evas_vg_shape_equal_commands(Eo *obj, const Eo *with)
 EAPI Efl_Canvas_Vg_Node*
 evas_vg_shape_add(Efl_Canvas_Vg_Node *parent)
 {
+   /* Warn it because the usage has been changed.
+      We can remove this message after v1.21. */
+   if (!parent) CRI("Efl_VG Shape doesn't allow null parent!");
    return efl_add(EFL_CANVAS_VG_SHAPE_CLASS, parent);
 }