style: Support full <image> syntax in list-style-image as per spec.

This allows supporting image-set(), etc, and simplifies the bullet frame
code significantly, too thanks to two changes:

  * Instead of manually managing the image request, use the CSS image
    loader, with the `REQUEST_REQUIRES_REFLOW` flag, to handle image
    loads correctly. This didn't exist when this code was initially
    implemented, but we can nicely use it now.

  * Instead of re-implementing another WebRender command-builder thing,
    we can just reuse the nsImageRenderer code.

Differential Revision: https://phabricator.services.mozilla.com/D100774

components/style/values/computed/url.rs

@@ -13,6 +13,3 @@ pub use crate::servo::url::{ComputedImageUrl, ComputedUrl};
 
 /// Computed <url> | <none>
 pub type UrlOrNone = GenericUrlOrNone<ComputedUrl>;
-
-/// Computed image <url> | <none>
-pub type ImageUrlOrNone = GenericUrlOrNone<ComputedImageUrl>;