Revert "app: [Windows] don't draw after WM_DESTROY destroyed the window"

This reverts commit 635df374952019ff8d274646ea9ce040744daa0f because it
didn't fix #603 after all.

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

app/os_windows.go

@@ -563,7 +563,8 @@ func (w *window) runLoop() {
 	msg := new(windows.Msg)
 loop:
 	for {
-		if w.hwnd != 0 && w.animating && !windows.PeekMessage(msg, 0, 0, 0, windows.PM_NOREMOVE) {
+		anim := w.animating
+		if anim && !windows.PeekMessage(msg, 0, 0, 0, windows.PM_NOREMOVE) {
 			w.draw(false)
 			continue
 		}