style: Support ray() in offset-path and make it animatable.

1. Add `generics::motion::OffsetPath`, and use specified `Angle` and
   computed `Angle` to define specified `OffsetPath` and computed `OffsetPath`.
2. Add `ray` function into `OffsetPath`.

We also tweak the degree from 150deg to 135deg in wpt (e.g.
offset-path-ray-001.html and others) to avoid floating point precision issues.
For example:
```
// offset-path: ray(150deg ...);
// offset-distance: 20px;
matrix:
{
  {0.500000 0.866025 0.000000 0.000000},
  {-0.866025 0.500000 0.000000 0.000000},
  {0.000000 0.000000 1.000000 0.000000},
  {10.000000 17.320509 0.000000 1.000000}
}

// rotate(60deg) translate(20px)
matrix:
{
  {0.500000 0.866025 0.000000 0.000000},
  {-0.866025 0.500000 0.000000 0.000000},
  {0.000000 0.000000 1.000000 0.000000},
  {10.000000 17.320507 0.000000 1.000000}
}
```
Their translate parts, 17.320509 vs 17.320507, are almost the same (only
tiny difference), which may cause the reftest failed.

Differential Revision: https://phabricator.services.mozilla.com/D42721

components/style/values/generics/mod.rs

@@ -29,6 +29,7 @@ pub mod font;
 pub mod grid;
 pub mod image;
 pub mod length;
+pub mod motion;
 pub mod position;
 pub mod rect;
 pub mod size;