Auto merge of #15905 - mephisto41:add-border-gradient, r=emilio

Add gradient border support.

webrender add gradient border support in https://github.com/servo/webrender/pull/953. This pr add support in servo.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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- [X] These changes do not require tests because it should be covered by wpt

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -755,6 +755,18 @@
      {}
     ]
    ],
+   "css/border-image-linear-gradient.html": [
+    [
+     "/_mozilla/css/border-image-linear-gradient.html",
+     [
+      [
+       "/_mozilla/css/border-image-linear-gradient-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/border_black_ridge_a.html": [
     [
      "/_mozilla/css/border_black_ridge_a.html",
@@ -6993,6 +7005,11 @@
      {}
     ]
    ],
+   "css/border-image-linear-gradient-ref.html": [
+    [
+     {}
+    ]
+   ],
    "css/border_black_solid.html": [
     [
      {}
@@ -20351,6 +20368,14 @@
    "d0bf8fafec5ff2c0cfde8f0d47083ca23b745588",
    "support"
   ],
+  "css/border-image-linear-gradient-ref.html": [
+   "3c1b61477c8a3cb7befc3ab81b80a128b142e3f1",
+   "support"
+  ],
+  "css/border-image-linear-gradient.html": [
+   "40ea388a8a920eaecb2d0d12f998ed60ac3b56e8",
+   "reftest"
+  ],
   "css/border_black_groove.html": [
    "42912352ee8f09ae0ddbac3dce35294dc8608bbe",
    "reftest_node"