[css-images] gradient interpolation hint

All,

the CSS images level 4 spec defines the color interpolation hint to move
the midpoint color between 2 color stops [1].
This was recently added to WebKit [2], Blink [3] and Firefox [4].  Since we
now have 3 implementations, can this be added to level 3?


1: http://dev.w3.org/csswg/css-images/#color-interpolation-hint
2: https://bugs.webkit.org/show_bug.cgi?id=137171
3: https://code.google.com/p/chromium/issues/detail?id=420527
4: https://bugzilla.mozilla.org/show_bug.cgi?id=1074056