Re: [css-images][svg2] gradient rendering and the image-rendering property

I am quite sympathetic to the argument that we should just make gradient
rendering good everywhere.

But in practice, Chromium at least is making visible
performance-over-quality tradeoffs, displaying banding in smooth gradients
and jagged pixelation on sharp transitions (in larger blocks than the
actual screen resolution).  Banding can also appear in subtle gradients in
any rendering tool that doesn't do explicit dithering, which can be an
issue with high-quality printing and SVG.

If we create a new `gradient-rendering` property (instead of re-purposing
`image-rendering`), that solves 2 issues:

   - User agents that can't control gradient rendering can simply indicate
   that they don't support certain values.  So an `@supports
   (gradient-rendering: smooth) {}` rule would fail if the renderer always
   applies some pixel aliasing to gradients, even if they have no problem with
   smooth image scaling.

   - The keywords can be chosen to make more sense for gradients instead of
   for image scaling.  The distinction between `crisp-edges` and `pixelated`
   is perhaps too confusing to factor in ( I had suggested that `crisp-edges`
   would alias to device pixels only for strict vertical and horizontal
   gradients, and use smoothing for other angles. )  And perhaps an explicit
   `dithered` keyword may prove useful for some implementations.

Like the other *-rendering properties, these would be a "hint" from the
author to the user agent; no one would be deemed non-conforming for failing
to provide advanced rendering control.

For reference, here are current behaviors, based on a quick test of the
following two pages on a Windows laptop with a moderately good screen
resolution:

   - CSS gradients at various angles:
   https://jsfiddle.net/987q718v/
   - SVG gradient (center) and CSS gradient:

   http://oreillymedia.github.io/SVG_Colors_Patterns_Gradients/ch07-positioningGradients-files/css-gradient-direction.html

Chrome: visible banding, sharp diagonal transitions aliased to ~2x2 pixel
blocks
Edge & Firefox: very subtle banding on the black-to-white gradients,
diagonal transitions aliased to device pixels.

I couldn't recreate the noticeable blurring on horizontal/vertical
gradients in Firefox that I've seen designers complain about previously.
Not sure if that means it's fixed or if certain other features of the
gradient are required to provoke it.