Glass drag image should be able to custom render it if the native platform can't provide the Image representation

On Mac we use [[[NSImage alloc] initWithPasteboard:pasteboard] autorelease] to ask the Pasteboard for the image if ([NSImage canInitWithPasteboard:pasteboard] == YES), but it's possible that the Pasteboard does not know how to render its contents.

Assuming JFX does not provide the API for clients to set their own image representation and Glass is asked to provide such image automatically, we should provide some image. For complex/unknown UTFs types we could always provide some template image representing drag, such as blank document icon.