Implement Specification from CSS Images 3.

Improvements:

1. Stops with positions less than preceding stops are moved.
     A common pattern is blue `70%, white 0` for sharp transitions.
2. Stop runs are correct if first stop has a position.
    The list `black 0%, red, gold` now renders the same as `black, red, gold`.
    Other runs may also be corrected.
3. Absolute length are no longer capped to 100%.