style: Implement image-rendering: smooth and image-rendering: pixelated

Also, more directly go from StyleImageRendering to wr::ImageRendering.

 * image-rendering: smooth the non-deprecated version of
   OptimizeQuality, which maps to SamplingFilter::LINEAR /
   wr::ImageRendering::Auto (which uses gl::LINEAR).

 * image-rendering: pixelated maps to wr::ImageRendering::Pixelated /
   SamplingFilter::POINT which is the same crisp-edges does.

Note that this uncovers that we were mapping image-rendering:
crisp-edges to wr::ImageRendering::Pixelated.

I'm going to preserve behavior on this patch but we should consider
switching that to map to wr::ImageRendering::CrispEdges on a
follow-up (filed bug 1728831 for this).

Differential Revision: https://phabricator.services.mozilla.com/D124378

components/style/values/specified/image.rs

@@ -1247,8 +1247,10 @@ impl MozImageRect {
 #[repr(u8)]
 pub enum ImageRendering {
     Auto,
+    Smooth,
     #[parse(aliases = "-moz-crisp-edges")]
     CrispEdges,
+    Pixelated,
     // From the spec:
     //
     //     This property previously accepted the values optimizeSpeed and
@@ -1256,10 +1258,9 @@ pub enum ImageRendering {
     //     them as valid values but must treat them as having the same behavior
     //     as crisp-edges and smooth respectively, and authors must not use
     //     them.
+    //
     #[cfg(feature = "gecko")]
     Optimizespeed,
     #[cfg(feature = "gecko")]
     Optimizequality,
-    #[cfg(feature = "servo")]
-    Pixelated,
 }