O(ericwilligers) Support additive animation for path strings crbug.com/699308

// Ray paths compose.
test_composition({
  property: 'offset-path',
  underlying: 'ray(20deg sides)',
  addFrom: 'ray(10deg sides)',
  addTo: 'ray(20deg sides)',
}, [
  {at: -0.3, expect: 'ray(27deg sides)'},
  {at: 0, expect: 'ray(30deg sides)'},
  {at: 0.3, expect: 'ray(33deg sides)'},
  {at: 0.6, expect: 'ray(36deg sides)'},
  {at: 1, expect: 'ray(40deg sides)'},
  {at: 1.5, expect: 'ray(45deg sides)'},
]);

// Ray paths witho