servo: Merge #18364 - Don't allow interpolating SVGPaintKind::None (from mantaroh:svgpaintkind-interpolation); r=birtles

This is a PR for https://bugzilla.mozilla.org/show_bug.cgi?id=1396483

PR #18103 disallowed interpolation between fill:none and fill:none, but that change was regressed by the refactoring in PR #18239.
This patch restores the intended behavior, disabling animation of SVGPaintKind::None.

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- [ x ] `./mach test-tidy` does not report any errors

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There are already these tests in dom/smil/tests of gecko, this PR will enable these tests.
For detail, see https://bugzilla.mozilla.org/show_bug.cgi?id=1396483 .

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Source-Repo: https://github.com/servo/servo
Source-Revision: e5efbeea6e1027931bf6d6f3cb429a00ea05bdec

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 95705d1eeb326353da770e8b8fd2887a8a3b07e1

servo/components/style/values/generics/svg.rs

@@ -35,6 +35,7 @@ pub struct SVGPaint<ColorType, UrlPaintServer> {
 #[derive(ToAnimatedValue, ToAnimatedZero, ToComputedValue, ToCss)]
 pub enum SVGPaintKind<ColorType, UrlPaintServer> {
     /// `none`
+    #[animation(error)]
     None,
     /// `<color>`
     Color(ColorType),