loadImage
Loads an image from a given URL. If the server referenced by the URL already has too many requests pending, this function will instead return undefined, indicating that the request should be retried later.
Return
A promise for the image that will resolve when the image is available, or undefined if there are too many active requests to the server, and the request should be retried later.
Parameters
imageryProvider
The imagery provider for the URL.
url
The URL of the image.