backend-vnc: make sure to finish frames with timestamps in the past

Round up the ms delay to make sure that the finish_frame_timer always
expires after the next frame_time. That way, finish_frame_handler()
never passes a timestamp in the future to weston_output_finish_frame().
Setting frame_time into the future risks hitting an assert in
weston_output_finish_frame(), when it is called from start_repaint_loop
within the frame interval.

Use CLIP() to simplify limiting the ms delay to a reasonable range.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>