GraphicFill loading state when writing style

[KaiVolland]

As stated by @nevaldas the loading state of images can be a problem when writing GraphicFill styleFunctions with external graphics: #388 (comment)

Possible solutions could be:

• Wait for the images to be loaded before returning the style. This could be difficult when using placeholders in the image url.
• optionaly passing the olmap and add an onload listener to rerender it
• …

Please discuss and add more ideas. :)

[jansule]

Also we could provide some kind of event, so the actual application can decide when to trigger the rerender. However, all applications would have to implement this then.