app: destroy GPU before ack'ing the DestroyEvent

The DestroyEvent ack allows the backend to release the window.
Ensure that the GPU associated with the window is done before that
happens.

Updates gio#67

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

app/window.go

@@ -200,6 +200,7 @@ func (w *Window) waitAck() {
 // Prematurely destroy the window and wait for the native window
 // destroy event.
 func (w *Window) destroy(err error) {
+	w.destroyGPU()
 	// Ack the current event.
 	w.ack <- struct{}{}
 	w.out <- system.DestroyEvent{Err: err}
@@ -211,6 +212,13 @@ func (w *Window) destroy(err error) {
 	}
 }
 
+func (w *Window) destroyGPU() {
+	if w.gpu != nil {
+		w.gpu.Release()
+		w.gpu = nil
+	}
+}
+
 func (w *Window) run(opts *window.Options) {
 	defer close(w.in)
 	defer close(w.out)
@@ -309,6 +317,7 @@ func (w *Window) run(opts *window.Options) {
 			case driverEvent:
 				w.driver = e2.driver
 			case system.DestroyEvent:
+				w.destroyGPU()
 				w.out <- e2
 				w.ack <- struct{}{}
 				return