wayland: Properly support changing of surface scale

wayland: Properly support changing of surface scale

When the preferred surface scale changes, for example when entering a
wl_output with a higher scale than any previous entered output, recreate
the shm surface and redraw the window content with the new window scale.

Before this patch, the internal scale would be changed, but the shm
surface would not be recreated given the new scale, i.e. we'd attach a
buffer for a different scale than wl_surface.set_scale specified.

https://bugzilla.gnome.org/show_bug.cgi?id=745076