render: add explicit sync API

include/wlr/render/wlr_renderer.h

@@ -45,6 +45,12 @@ struct wlr_renderer {
 		 * Does the renderer support color transforms on its output?
 		 */
 		bool output_color_transform;
+		/**
+		 * Whether wait/signal timelines are supported.
+		 *
+		 * See struct wlr_drm_syncobj_timeline.
+		 */
+		bool timeline;
 	} features;
 
 	// private state