screencast: use refcounting to restart wlroots loop after reneg

this complements the previous commit, which won't restart the wlroots
loop after renegotiation. Restarting the stream on the first new
add_buffer call works, but maybe we can find a nicer solution here.

include/screencast_common.h

@@ -92,6 +92,7 @@ struct xdpw_screencast_instance {
 	uint32_t node_id;
 	bool pwr_stream_state;
 	struct pw_buffer *current_pw_buffer;
+	int buffer_refcount;
 
 	// wlroots
 	struct zwlr_screencopy_frame_v1 *frame_callback;

src/screencast/pipewire_screencast.c

@@ -72,7 +72,7 @@ static void pwr_handle_stream_state_changed(void *data,
 	switch (state) {
 	case PW_STREAM_STATE_STREAMING:
 		cast->pwr_stream_state = true;
-		xdpw_wlr_frame_start(cast);
+		xdpw_wlr_register_cb(cast);
 		break;
 	default:
 		cast->pwr_stream_state = false;
@@ -156,6 +156,12 @@ static void pwr_handle_stream_add_buffer(void *data, struct pw_buffer *buffer) {
 		// create wl_buffer
 		d[0].data = import_shm_buffer(cast, d[0].fd, cast->simple_frame.format,
 			cast->simple_frame.width, cast->simple_frame.height, cast->simple_frame.stride);
+
+		cast->buffer_refcount++;
+	}
+	// TODO: start stream again after renegotiation
+	if (cast->pwr_stream_state && cast->buffer_refcount == 1) {
+		xdpw_wlr_frame_start(cast);
 	}
 }