Intent to prototype and ship: image-rendering: smooth and image-rendering: pixelated

[Emilio Cobos Álvarez]

Summary: Implement image-rendering: pixelated and image-rendering:
smooth to tweak image scaling algorithms.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=856337

Standard: https://drafts.csswg.org/css-images/#propdef-image-rendering

Preference: N/A, since we had support for ~most of this already behind
other values.

DevTools bug: N/A

Other browsers: Both WebKit and Blink support "pixelated". I implemented
"smooth" drive-by since it's a non-deprecated alias to
"optimizeQuality", which we already supported.

web-platform-tests: There were existing tests that started passing
there, see the patch for details.

-- Emilio