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/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",