MetaWindowActor: unset the shaped texture overlay path for frameless windows

When a window loses its frame we must unset any overlay path previously set on
the shaped texture.

Not doing so would cause rendering glitches near the window corners in
e.g. chrome/chromium by changing the Appearance preference "Use system title
bar and borders" → "Hide system title bar and use compact borders".

https://bugzilla.gnome.org/show_bug.cgi?id=659477