xwayland: Punt stdout / stderr to /dev/null

This should really go to the journal, but I'm tired of seeing Xorg
log a bunch of garbage that I don't care about when debugging mutter.
wayland-surface: Unref the buffer after removing the destroy listener
surface-actor-wayland: Make sure to clean up on dispose

We need to remove our destroy handler if the surface is destroyed
before the buffer is, which is the case when we have no destroy
effect.