Auto merge of #16319 - canaltinova:element, r=upsuper

stylo: Add -moz-element support

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

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<!-- 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)

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components/style/gecko/conversions.rs

@@ -11,7 +11,7 @@
 use app_units::Au;
 use gecko::values::{convert_rgba_to_nscolor, GeckoStyleCoordConvertible};
 use gecko_bindings::bindings::{Gecko_CreateGradient, Gecko_SetGradientImageValue, Gecko_SetUrlImageValue};
-use gecko_bindings::bindings::Gecko_InitializeImageCropRect;
+use gecko_bindings::bindings::{Gecko_InitializeImageCropRect, Gecko_SetImageElement};
 use gecko_bindings::structs::{nsStyleCoord_CalcValue, nsStyleImage};
 use gecko_bindings::structs::{nsresult, SheetType};
 use gecko_bindings::sugar::ns_style_coord::{CoordDataValue, CoordDataMut};
@@ -141,6 +141,11 @@ impl nsStyleImage {
                     image_rect.left.to_gecko_style_coord(&mut rect.data_at_mut(3));
                 }
             }
+            Image::Element(ref element) => {
+                unsafe {
+                    Gecko_SetImageElement(self, element.as_ptr());
+                }
+            },
             _ => (),
         }
     }