app: release GPU before processing DestroyEvent

In fact, move the release of the GPU to a deferred function, saving
a release.

Fixes #65

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

app/window.go

@@ -218,6 +218,12 @@ func (w *Window) run(opts *window.Options) {
 		w.out <- system.DestroyEvent{Err: err}
 		return
 	}
+	defer func() {
+		if w.gpu != nil {
+			w.gpu.Release()
+			w.gpu = nil
+		}
+	}()
 	for {
 		var timer <-chan time.Time
 		if w.delayedDraw != nil {
@@ -297,8 +303,6 @@ func (w *Window) run(opts *window.Options) {
 				}
 				if e2.Sync {
 					if err := w.gpu.Flush(); err != nil {
-						w.gpu.Release()
-						w.gpu = nil
 						w.destroy(err)
 						return
 					}