Auto merge of #12651 - heycam:background-position, r=Manishearth

Add support for background-position in geckolib.

<!-- Please describe your changes on the following line: -->

r? @Manishearth

---
<!-- 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
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because this is a geckolib-only change

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12651)
<!-- Reviewable:end -->

components/style/gecko_conversions.rs

@@ -8,7 +8,7 @@
 
 use app_units::Au;
 use gecko_bindings::structs::nsStyleCoord_CalcValue;
-use values::computed::CalcLengthOrPercentage;
+use values::computed::{CalcLengthOrPercentage, LengthOrPercentage};
 
 impl From<CalcLengthOrPercentage> for nsStyleCoord_CalcValue {
     fn from(other: CalcLengthOrPercentage) -> nsStyleCoord_CalcValue {
@@ -34,3 +34,35 @@ impl From<nsStyleCoord_CalcValue> for CalcLengthOrPercentage {
         }
     }
 }
+
+impl From<LengthOrPercentage> for nsStyleCoord_CalcValue {
+    fn from(other: LengthOrPercentage) -> nsStyleCoord_CalcValue {
+        match other {
+            LengthOrPercentage::Length(au) => {
+                nsStyleCoord_CalcValue {
+                    mLength: au.0,
+                    mPercent: 0.0,
+                    mHasPercent: false,
+                }
+            },
+            LengthOrPercentage::Percentage(pc) => {
+                nsStyleCoord_CalcValue {
+                    mLength: 0,
+                    mPercent: pc,
+                    mHasPercent: true,
+                }
+            },
+            LengthOrPercentage::Calc(calc) => calc.into(),
+        }
+    }
+}
+
+impl From<nsStyleCoord_CalcValue> for LengthOrPercentage {
+    fn from(other: nsStyleCoord_CalcValue) -> LengthOrPercentage {
+        match (other.mHasPercent, other.mLength) {
+            (false, _) => LengthOrPercentage::Length(Au(other.mLength)),
+            (true, 0) => LengthOrPercentage::Percentage(other.mPercent),
+            _ => LengthOrPercentage::Calc(other.into()),
+        }
+    }
+}

components/style/properties/gecko.mako.rs

@@ -922,7 +922,8 @@ fn static_assert() {
 // add support for parsing these lists in servo and pushing to nsTArray's.
 <% skip_background_longhands = """background-color background-repeat
                                   background-image background-clip
-                                  background-origin background-attachment""" %>
+                                  background-origin background-attachment
+                                  background-position""" %>
 <%self:impl_trait style_struct_name="Background"
                   skip_longhands="${skip_background_longhands}"
                   skip_additionals="*">
@@ -1010,6 +1011,29 @@ fn static_assert() {
         };
     }
 
+    pub fn copy_background_position_from(&mut self, other: &Self) {
+        self.gecko.mImage.mPositionXCount = cmp::min(1, other.gecko.mImage.mPositionXCount);
+        self.gecko.mImage.mPositionYCount = cmp::min(1, other.gecko.mImage.mPositionYCount);
+        self.gecko.mImage.mLayers.mFirstElement.mPosition =
+            other.gecko.mImage.mLayers.mFirstElement.mPosition;
+    }
+
+    pub fn clone_background_position(&self) -> longhands::background_position::computed_value::T {
+        let position = &self.gecko.mImage.mLayers.mFirstElement.mPosition;
+        longhands::background_position::computed_value::T {
+            horizontal: position.mXPosition.into(),
+            vertical: position.mYPosition.into(),
+        }
+    }
+
+    pub fn set_background_position(&mut self, v: longhands::background_position::computed_value::T) {
+        let position = &mut self.gecko.mImage.mLayers.mFirstElement.mPosition;
+        position.mXPosition = v.horizontal.into();
+        position.mYPosition = v.vertical.into();
+        self.gecko.mImage.mPositionXCount = 1;
+        self.gecko.mImage.mPositionYCount = 1;
+    }
+
     pub fn copy_background_image_from(&mut self, other: &Self) {
         unsafe {
             Gecko_CopyImageValueFrom(&mut self.gecko.mImage.mLayers.mFirstElement.mImage,