app: ignore app.Size when in fullscreen mode

Setting the window size while in fulscreen mode does not make sense.

Fixes gio#220

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

app/os_windows.go

@@ -496,7 +496,7 @@ func (w *window) Configure(options []Option) {
 	cnf := w.config
 	cnf.apply(cfg, options)
 
-	if prev.Size != cnf.Size {
+	if cnf.Mode != Fullscreen && prev.Size != cnf.Size {
 		width := int32(cnf.Size.X)
 		height := int32(cnf.Size.Y)
 		w.config.Size = cnf.Size