style: Define offset-path and implement it in style system.

Define OffsetPath & SVGPathData on the servo-side, and StyleMotion &
StyleSVGPath on the gecko-side. We parse the SVG Path string into a
vector of PathCommand. To build the gfx::Path, we will convert it into
gfx::Path later in a different patch.

The basic flow is:
  - Parse SVG Path String into SVGPathData (in Rust).
  - Use cbindgen to make sure the layout of PathCommand and StylePathCommand, and then set the Box[PathCommand] into nsTArray<StylePathCommand>.
  - Try to convert nsTArray<StylePathCommand> into gfx::Path. (This part will be implemented in a different patch.)

Finally, we use the gfx::Path to create a motion path transform.
The layout implementation is in the later patch.

Depends on D2962

Differential Revision: https://phabricator.services.mozilla.com/D2963

components/style/properties/gecko.mako.rs

@@ -3053,7 +3053,7 @@ fn static_assert() {
                           scroll-snap-points-x scroll-snap-points-y
                           scroll-snap-type-x scroll-snap-type-y scroll-snap-coordinate
                           perspective-origin -moz-binding will-change
-                          overscroll-behavior-x overscroll-behavior-y
+                          offset-path overscroll-behavior-x overscroll-behavior-y
                           overflow-clip-box-inline overflow-clip-box-block
                           perspective-origin -moz-binding will-change
                           shape-outside contain touch-action translate
@@ -3681,6 +3681,51 @@ fn static_assert() {
     ${impl_simple_copy("contain", "mContain")}
 
     ${impl_simple_type_with_conversion("touch_action")}
+
+    pub fn set_offset_path(&mut self, v: longhands::offset_path::computed_value::T) {
+        use gecko_bindings::bindings::{Gecko_NewStyleMotion, Gecko_NewStyleSVGPath};
+        use gecko_bindings::bindings::Gecko_SetStyleMotion;
+        use gecko_bindings::structs::StyleShapeSourceType;
+        use values::specified::OffsetPath;
+
+        let motion = unsafe { Gecko_NewStyleMotion().as_mut().unwrap() };
+        match v {
+            OffsetPath::None => motion.mOffsetPath.mType = StyleShapeSourceType::None,
+            OffsetPath::Path(servo_path) => {
+                motion.mOffsetPath.mType = StyleShapeSourceType::Path;
+                let gecko_path = unsafe {
+                    let ref mut source = motion.mOffsetPath;
+                    Gecko_NewStyleSVGPath(source);
+                    &mut source.__bindgen_anon_1.mSVGPath.as_mut().mPtr.as_mut().unwrap().mPath
+                };
+                unsafe { gecko_path.set_len(servo_path.commands().len() as u32) };
+                debug_assert_eq!(gecko_path.len(), servo_path.commands().len());
+                for (servo, gecko) in servo_path.commands().iter().zip(gecko_path.iter_mut()) {
+                    // unsafe: cbindgen ensures the representation is the same.
+                    *gecko = unsafe { transmute(*servo) };
+                }
+            },
+        }
+        unsafe { Gecko_SetStyleMotion(&mut self.gecko.mMotion, motion) };
+    }
+
+    pub fn clone_offset_path(&self) -> longhands::offset_path::computed_value::T {
+        use values::specified::OffsetPath;
+        match unsafe { self.gecko.mMotion.mPtr.as_ref() } {
+            None => OffsetPath::none(),
+            Some(v) => (&v.mOffsetPath).into()
+        }
+    }
+
+    pub fn copy_offset_path_from(&mut self, other: &Self) {
+        use gecko_bindings::bindings::Gecko_CopyStyleMotions;
+        unsafe { Gecko_CopyStyleMotions(&mut self.gecko.mMotion, other.gecko.mMotion.mPtr) };
+    }
+
+    pub fn reset_offset_path(&mut self, other: &Self) {
+        self.copy_offset_path_from(other);
+    }
+
 </%self:impl_trait>
 
 <%def name="simple_image_array_property(name, shorthand, field_name)">

components/style/properties/longhands/box.mako.rs

@@ -356,6 +356,17 @@ ${helpers.predefined_type(
     servo_restyle_damage="reflow_out_of_flow"
 )}
 
+// Motion Path Module Level 1
+${helpers.predefined_type(
+    "offset-path",
+    "OffsetPath",
+    "computed::OffsetPath::none()",
+    animation_value_type="none",
+    gecko_pref="layout.css.motion-path.enabled",
+    flags="CREATES_STACKING_CONTEXT FIXPOS_CB",
+    spec="https://drafts.fxtf.org/motion-1/#offset-path-property"
+)}
+
 // CSSOM View Module
 // https://www.w3.org/TR/cssom-view-1/
 ${helpers.single_keyword("scroll-behavior",