compositor-drm: Wait for vblank events before starting next frame

Besides the fact of the frame not being done, assigning planes before
the vblank handler is called will make the state inconsistent, leading
to a crash.

src/compositor-drm.c

@@ -98,6 +98,9 @@ struct drm_output {
 	uint32_t connector_id;
 	drmModeCrtcPtr original_crtc;
 
+	int vblank_pending;
+	int page_flip_pending;
+
 	struct gbm_surface *surface;
 	struct gbm_bo *cursor_bo[2];
 	int current_cursor;
@@ -118,6 +121,8 @@ struct drm_sprite {
 	struct weston_surface *surface;
 	struct weston_surface *pending_surface;
 
+	struct drm_output *output;
+
 	struct drm_compositor *compositor;
 
 	struct wl_listener destroy_listener;
@@ -358,6 +363,8 @@ drm_output_repaint(struct weston_output *output_base,
 		return;
 	}
 
+	output->page_flip_pending = 1;
+
 	/*
 	 * Now, update all the sprite surfaces
 	 */
@@ -391,6 +398,9 @@ drm_output_repaint(struct weston_output *output_base,
 			weston_log("vblank event request failed: %d: %s\n",
 				ret, strerror(errno));
 		}
+
+		s->output = output;
+		output->vblank_pending = 1;
 	}
 
 	return;
@@ -402,6 +412,10 @@ vblank_handler(int fd, unsigned int frame, unsigned int sec, unsigned int usec,
 {
 	struct drm_sprite *s = (struct drm_sprite *)data;
 	struct drm_compositor *c = s->compositor;
+	struct drm_output *output = s->output;
+	uint32_t msecs;
+
+	output->vblank_pending = 0;
 
 	if (s->surface) {
 		weston_buffer_post_release(s->surface->buffer);
@@ -420,6 +434,11 @@ vblank_handler(int fd, unsigned int frame, unsigned int sec, unsigned int usec,
 		s->fb_id = s->pending_fb_id;
 		s->pending_fb_id = 0;
 	}
+
+	if (!output->page_flip_pending) {
+		msecs = sec * 1000 + usec / 1000;
+		weston_output_finish_frame(&output->base, msecs);
+	}
 }
 
 static void
@@ -429,6 +448,8 @@ page_flip_handler(int fd, unsigned int frame,
 	struct drm_output *output = (struct drm_output *) data;
 	uint32_t msecs;
 
+	output->page_flip_pending = 0;
+
 	if (output->current) {
 		if (output->current->is_client_buffer)
 			gbm_bo_destroy(output->current->bo);
@@ -440,8 +461,10 @@ page_flip_handler(int fd, unsigned int frame,
 	output->current = output->next;
 	output->next = NULL;
 
+	if (!output->vblank_pending) {
 		msecs = sec * 1000 + usec / 1000;
 		weston_output_finish_frame(&output->base, msecs);
+	}
 }
 
 static int