These allow you to specify the point were the gradient transitions
from one color to the next (without a transition hint the transition
occurs at the point 50% of the way between the two colors).
There is a little bit of guesswork in this implementation as the
specification left out how hints work with the color stop fixup,
though it appears that they are treated the same as color stops.
This commit moves both the ImageStyleValue and LinearGradientStyleValue
to a common base class of AbstractImageStyleValue. This abstracts
getting the natural_width/height, loading/resolving, and painting
the image.
Now for 'free' you get:
- Linear gradients working with the various background sizing/repeat
properties.
- Linear gradients working as list-markers :^) -- best feature ever!
P.s. This commit is a little large as it's tricky to make this change
incrementally without breaking things.
This implements support for painting linear-gradients in a spec
correct way :^).
Right now it supports:
- Multi-stop gradients
- Color stop fixups
- Using pre-multiplied alpha mixing when required
- Painting gradients at arbitrary angles
It still needs to support:
- Transition hints
- Double position color stops
However what is implemented now seems to be accurate to other
browsers, and covers the most common use cases.
