shaped-texture: Invalidate mask texture when main texture changes size

We currently rely only on MetaWindowActor to update the mask
texture. This isn't good enough since we might get asked to use the
mask (e.g. via meta_shaped_texture_get_image() ) after having a new
texture size but before MetaWindowActor decides to update the mask in
which case we might crash since cogl_texture_new_from_sub_texture()
might fail with an early return such as

Cogl-CRITICAL **: cogl_sub_texture_new: assertion 'sub_x + sub_width
<= next_width' failed

https://bugzilla.gnome.org/show_bug.cgi?id=762639

src/compositor/meta-shaped-texture.c

@@ -303,6 +303,7 @@ set_cogl_texture (MetaShapedTexture *stex,
     {
       priv->tex_width = width;
       priv->tex_height = height;
+      meta_shaped_texture_set_mask_texture (stex, NULL);
       clutter_actor_queue_relayout (CLUTTER_ACTOR (stex));
       g_signal_emit (stex, signals[SIZE_CHANGED], 0);
     }