app: don't delay FrameEvent.Frame by v-sync latency

We should return as soon as possible from FrameEvent.Frame to
allow the main goroutine to continue processing other tasks.
Whereas GPU.Frame may touch the frame ops, GPU.Present will not,
so this change moves Present to after returning from FrameEvent.Frame.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/window.go

@@ -248,10 +248,7 @@ func (w *Window) render(frame *op.Ops, viewport image.Point) error {
 	if err != nil {
 		return err
 	}
-	if err := w.gpu.Frame(frame, target, viewport); err != nil {
-		return err
-	}
-	return w.ctx.Present()
+	return w.gpu.Frame(frame, target, viewport)
 }
 
 func (w *Window) processFrame(d driver, frameStart time.Time) {
@@ -808,6 +805,9 @@ func (w *Window) processEvent(d driver, e event.Event) {
 			// We're done with frame, let the client continue.
 			w.frameAck <- struct{}{}
 		}
+		if err == nil && w.gpu != nil {
+			err = w.ctx.Present()
+		}
 		if err != nil {
 			w.destroyGPU()
 			w.out <- system.DestroyEvent{Err: err}