preview-widget: Remove meta_preview_get_clip_region

The concept of a clip region doesn't make sense now that we have anti-aliased
corners and a full alpha channel. Once the theme transition is complete,
creating a preview image with an alpha channel will be possible by passing
an ARGB surface to gtk_widget_draw(preview_widget, ...);

https://bugzilla.gnome.org/show_bug.cgi?id=676052