output: stop assuming a frame is pending in init

- The DRM backend initially doesn't have a frame scheduled initially.
  However the compositor is expected to set a mode to start the
  rendering loop (frame_pending is set to true in drm_crtc_pageflip).
- The headless and X11 backends have a timer to schedule frames, so they
  ignore this hint completely.
- The Wayland backend renders a fake frame to start the rendering loop.
  It's the only case where a frame is pending on init, move the
  assumption there.

backend/wayland/output.c

@@ -582,6 +582,7 @@ struct wlr_output *wlr_wl_output_create(struct wlr_backend *wlr_backend) {
 			NULL)) {
 		goto error;
 	}
+	wlr_output->frame_pending = true;
 
 	wl_list_insert(&backend->outputs, &output->link);
 	wlr_output_update_enabled(wlr_output, true);