src/output.c: restore flicker free resize when running nested

src/output.c: restore flicker free resize when running nested

PR #1301 did partly revert the first variant because
`lab_wlr_scene_output_commit()` (which uses the `output->pending`
state in contrast to `wlr_scene_output_commit()`) was deemd to
cause 'frozen' frames during video playback (#1273).

We are now back at using `lab_wlr_scene_output_commit()` for
unrelated reasons and the cause of the original issue #1273 was
likely that `wlr_scene_output_send_frame_done()` was only sent
on a successful commit. This was changed to always be sent.

So lets restore the flicker free nested resize as well.