WIP: renderer-native: Simplify swap notification

Instead of notifying when total_pending_flips == 0 in both on_crtc_flipped and meta_onscreen_native_flip_crtcs, we can do it just once in the function that's guaranteed to come next: flip_closure_destroyed.

This new form results in the same behaviour, but requires less code and is more robust in that it will soon support the possibility that on_crtc_flipped is never called (total_pending_flips never decremented). Because in future slower monitors may drop frames to keep up with faster ones.