In display_list_builder.rs handle cover and contain size keywords.

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%.

The property border-image-outset is not yet implemented.
Note: Also support repeating-linear-gradients for borders.

Render gradients where the last stop is the same position
as the previous one like Chrome and Firefox do.

The length should be the horizontal radius and not the
hypotenuse.

If the first stop of a non-repeating gradient is
not placed at offset 0.0 it is duplicated at this
position. This solves the problem of the first
stop being ignored if it is placed at the same
offset as the next stop.