app: ensure Invalidate can be invoked when window is closing

This commit ensures that it is safe to invoke Invalidate() from another goroutine
while a Gio window may be in the process of closing. It can be difficult to prevent
this from happening, as window handles can easily be managed by a type that doesn't
know the exact moment of window close (it might be waiting on the window event loop
to return, but that hasn't happened yet). Without this change, the nil window
driver results in a panic in this situation.

Co-authored-by: Chris Waldon <christopher.waldon.dev@gmail.com>
Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/window.go

@@ -387,6 +387,9 @@ func (w *Window) setNextFrame(at time.Time) {
 }
 
 func (c *callbacks) SetDriver(d driver) {
+	if d == nil {
+		panic("nil driver")
+	}
 	c.w.driver = d
 }
 
@@ -635,6 +638,9 @@ func (w *Window) processEvent(e event.Event) bool {
 		w.coalesced.frame = &e2
 	case DestroyEvent:
 		w.destroyGPU()
+		w.invMu.Lock()
+		w.mayInvalidate = false
+		w.invMu.Unlock()
 		w.driver = nil
 		if q := w.timer.quit; q != nil {
 			q <- struct{}{}