wayland: fix pointer focus for destroyed surfaces

We had an assertion in meta_wayland_surface_free() that after
a repick() we would not choose the freed surface, but that didn't
consider surfaces destroyed while holding the implicit pointer
grab (ie, because the user clicked on the X button). In that case,
we need to bypass the grab infrastructure and explicitly unfocus
the dead surface.

https://bugzilla.gnome.org/show_bug.cgi?id=706982