backend-headless: use weston_output_arm_frame_timer()
headless_output_repaint() is not actually called immediately after weston_output_finish_frame(): Weston waits for "frame-duration - repaint-window" but at least one millisecond. And renderning can also take an arbitrary amount of time.
So use weston_output_arm_frame_timer() like all other timer based backends. It takes all of this into account and calculates the delay relative to the next expected vblank time.