app: move ownership of GPU context to app.Window

The Window creates the context, and should also be responsible for
destroying it.

As a bonus, the wrong release ordering of loop.renderLoop is fixed.
Before this change, the context would be destroyed before the renderer
got a chance to destroy its resources.

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

app/loop.go

@@ -77,7 +77,6 @@ func (l *renderLoop) renderLoop(ctx window.Context) error {
 			return
 		}
 		defer g.Release()
-		defer ctx.Release()
 		initErr <- nil
 	loop:
 		for {