Asynchronously loads the given image URL.  Returns a promise that will resolve to
an 
    
    
    
    
    
        
Image once loaded, or reject if the image failed to load.
    | Name | Type | Default | Description | 
|---|---|---|---|
| url | String | The source URL of the image. | |
| allowCrossOrigin | Boolean | true | optional Whether to request the image using Cross-Origin Resource Sharing (CORS). CORS is only actually used if the image URL is actually cross-origin. Data URIs are never requested using CORS. | 
| request | Request | optional The request object. Intended for internal use only. | 
Returns:
    a promise that will resolve to the requested data when loaded. Returns undefined if 
        
    
    
request.throttle is true and the request does not have high enough priority.
Example:
// load a single image asynchronously
Cesium.loadImage('some/image/url.png').then(function(image) {
    // use the loaded image
}).otherwise(function(error) {
    // an error occurred
});
// load several images in parallel
when.all([loadImage('image1.png'), loadImage('image2.png')]).then(function(images) {
    // images is an array containing all the loaded images
});