[TIMOB-25280] Windows: Allow view.toImage to work on elements not in the XAML visual tree

Description

Currently the .toImage method for views will only work if the view is in the XAML visual tree, that is to say added to another view/window. I believe this is a limitation of the API used [rendertargetbitmap](https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.media.imaging.rendertargetbitmap), I think it might be possible to use [XamlRenderingBackgroundTask](https://docs.microsoft.com/en-us/uwp/api/Windows.UI.Xaml.Media.Imaging.XamlRenderingBackgroundTask) see https://stackoverflow.com/questions/29262195/rendertargetbitmap-without-putting-it-on-ui

var win = Ti.UI.createWindow();

var view = Ti.UI.createView({
    backgroundColor: 'red',
    width: 400,
    height: 400,
    borderRadius: 200
});
// win.add(view);
view.toImage(function (image) {
    console.log('image done');
    console.log(image.length)
});
win.open();