servo: Merge #16319 - stylo: Add -moz-element support (from canaltinova:element); r=upsuper

Implemented -moz-element for background property.
r=upsuper

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15443 and [Bug 1341761](https://bugzilla.mozilla.org/show_bug.cgi?id=1341761)

Source-Repo: https://github.com/servo/servo
Source-Revision: 3620e755a96d7272cfd28751696ec42ac0a0ce70

servo/components/layout/display_list_builder.rs

@@ -657,6 +657,9 @@ impl FragmentDisplayListBuilding for Fragment {
                 Some(computed::Image::ImageRect(_)) => {
                     // TODO: Implement `-moz-image-rect`
                 }
+                Some(computed::Image::Element(_)) => {
+                    // TODO: Implement `-moz-element`
+                }
             }
         }
     }
@@ -1154,6 +1157,9 @@ impl FragmentDisplayListBuilding for Fragment {
             Some(computed::Image::ImageRect(..)) => {
                 // TODO: Handle border-image with `-moz-image-rect`.
             }
+            Some(computed::Image::Element(..)) => {
+                // TODO: Handle border-image with `-moz-element`.
+            }
             Some(computed::Image::Url(ref image_url)) => {
                 if let Some(url) = image_url.url() {
                     let webrender_image = state.layout_context