MutterShapedWindow: drop all indirect references to textures

We don't get correct notifications for the ::cogl-texture property of
ClutterTexture in the case when we are unsetting the pixmap before calling
XFreePixmap. (This is because ClutterX11TexturePixmap is a hack on top
of ClutterTexture and we're a hack on top of that.) So we need to manually
clear everything out.

For consistency we also make sure that we drop all references to dead
textures:

 - When the shape changes
 - If the window pixmap texture changes without first being cleared
   (this is not expected to happen)

https://bugzilla.gnome.org/show_bug.cgi?id=627210