[CSS Images] syntax for color interpolation hint

http://dev.w3.org/csswg/css-images-4/#color-interpolation-hint

This feature lets authors adjust the interpolation between two color stops.
The current syntax uses a fake color stop with a position and no color as
an instruction to interpolate differently between the two real stops on
either side of it. The position of the color stop is used as a parameter to
control the interpolation.

I'm unconvinced this syntax is a good idea. Using a fake color stop for
this purpose just seems like a hack. If someone accidentally leaves out a
color from a color stop they will trigger this feature accidentally. It
also means there are edge cases we have to check for and reject in the
parser, e.g. consecutive fake color stops. It's also not particularly
extensible; we're consuming this part of the syntax (color-less stop) for a
particular feature that isn't directly related to that syntax.

For an obscure feature like this, can't we afford to be more explicit. e.g.
attach an interpolation() function to the color-stop before the interval we
want to change the interpolation of? Or allow interpolation() values to be
interspersed with color stops? Am I the only person who has qualms here?

Rob
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