WIP: feat: implement frame timing

Make frame_scheduler able to time frames, e.g. tell flutter when to
start rendering a frame when it receives a frame request.

For now, this just unconditionally replies with the current time, i.e.
starts rendering the frame immediately.

Author: ardera

src/frame_scheduler.c

@@ -95,11 +95,17 @@ void frame_scheduler_on_fl_vsync_request(struct frame_scheduler *scheduler, intp
     //
 
     /// TODO: Implement
-    /// For now, just unconditionally reply
+    /// For now, just unconditionally reply with the current time and a 60FPS frame interval.
     if (scheduler->present_mode == kTripleBufferedVsync_PresentMode) {
-        scheduler->vsync_cb(scheduler->userdata, vsync_baton, 0, 0);
+        uint64_t now = get_monotonic_time();
+        uint64_t now_plus_frame_interval = now + 1000000000/60;
+        scheduler->vsync_cb(scheduler->userdata, vsync_baton, now, now_plus_frame_interval);
     } else if (scheduler->present_mode == kDoubleBufferedVsync_PresentMode) {
-        scheduler->vsync_cb(scheduler->userdata, vsync_baton, 0, 0);
+        uint64_t now = get_monotonic_time();
+        uint64_t now_plus_frame_interval = now + 1000000000/60;
+        scheduler->vsync_cb(scheduler->userdata, vsync_baton, now, now_plus_frame_interval);
+    } else {
+        UNREACHABLE();
     }
 }