compositor: Emit frame_signal from weston_output_do_read_pixels()

This is the point where we have just finished rendering the new scene
but before we swap it to the front buffer.  At this point, the
output->previous_damage region exactly corresponds to what was just
renders, as compared to previous frame.

src/compositor.c

@@ -1142,8 +1142,7 @@ weston_output_finish_frame(struct weston_output *output, int msecs)
 		wl_display_get_event_loop(compositor->wl_display);
 	int fd;
 
-	wl_signal_emit(&output->frame_signal, &msecs);
-
+	output->frame_time = msecs;
 	if (output->repaint_needed) {
 		weston_output_repaint(output, msecs);
 		return;
@@ -2871,6 +2870,8 @@ weston_output_do_read_pixels(struct weston_output *output)
 {
 	struct weston_read_pixels *r, *next;
 
+	wl_signal_emit(&output->frame_signal, &output->frame_time);
+
 	glPixelStorei(GL_PACK_ALIGNMENT, 1);
 	wl_list_for_each_safe(r, next, &output->read_pixels_list, link) {
 		glReadPixels(r->x, r->y, r->width, r->height,

src/compositor.h

@@ -172,6 +172,7 @@ struct weston_output {
 	int dirty;
 	struct wl_list read_pixels_list;
 	struct wl_signal frame_signal;
+	uint32_t frame_time;
 
 	char *make, *model;
 	uint32_t subpixel;

src/screenshooter.c

@@ -275,11 +275,6 @@ weston_recorder_frame_notify(struct wl_listener *listener, void *data)
 	r = pixman_region32_rectangles(&damage, &n);
 	if (n == 0)
 		return;
-	if (recorder->count++ == 0)
-		/* The first callback gives us the frame immediately
-		 * before the weston_output_damage() call, and
-		 * typically doesn't give us a full frame of damage.*/
-		return;
 
 	header.msecs = msecs;
 	header.nrects = n;