Bug 1685078 - Support full <image> syntax in list-style-image as per spec. r=jrmuizel,TYLin

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

layout/generic/nsBlockFrame.cpp

@@ -7394,7 +7394,7 @@ bool nsBlockFrame::MarkerIsEmpty() const {
                "should only care when we have an outside ::marker");
   nsIFrame* marker = GetMarker();
   const nsStyleList* list = marker->StyleList();
-  return list->mCounterStyle.IsNone() && !list->GetListStyleImage() &&
+  return list->mCounterStyle.IsNone() && list->mListStyleImage.IsNone() &&
          marker->StyleContent()->ContentCount() == 0;
 }