app: don't touch the frame after returning from FrameEvent.Frame

It was already true that the GPU wouldn't touch the frame after
calling GPU.Draw. This change makes FrameEvent.Frame block until
GPU.Draw is complete.

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

app/window.go

@@ -36,6 +36,7 @@ type Window struct {
 	ack         chan struct{}
 	invalidates chan struct{}
 	frames      chan *op.Ops
+	frameAck    chan struct{}
 
 	stage        system.Stage
 	animating    bool
@@ -95,6 +96,7 @@ func NewWindow(options ...Option) *Window {
 		ack:         make(chan struct{}),
 		invalidates: make(chan struct{}, 1),
 		frames:      make(chan *op.Ops),
+		frameAck:    make(chan struct{}),
 	}
 	w.callbacks.w = w
 	go w.run(opts)
@@ -118,6 +120,7 @@ func (w *Window) Queue() *Queue {
 // the window state from previous calls.
 func (w *Window) update(frame *op.Ops) {
 	w.frames <- frame
+	<-w.frameAck
 }
 
 func (w *Window) draw(size image.Point, frame *op.Ops) {
@@ -279,17 +282,27 @@ func (w *Window) run(opts *window.Options) {
 					}
 				}
 				var frame *op.Ops
-				// Wait for either a frame or the ack event,
-				// which meant that the client didn't draw.
+				// Wait for either a frame or an ack event to go
+				// through, which means that the client didn't give us
+				// a frame.
+				gotFrame := false
 				select {
 				case frame = <-w.frames:
+					gotFrame = true
 				case w.out <- ackEvent:
 				}
 				if err != nil {
+					if gotFrame {
+						w.frameAck <- struct{}{}
+					}
 					w.destroy(err)
 					return
 				}
-				w.draw(e2.Size, frame)
+				if gotFrame {
+					w.draw(e2.Size, frame)
+					// We're done with frame, let the client continue.
+					w.frameAck <- struct{}{}
+				}
 				if e2.Sync {
 					if err := w.gpu.Flush(); err != nil {
 						w.gpu.Release()