Add support for lighter operator in feComposite

Spec: https://www.w3.org/TR/filter-effects-1/#attr-valuedef-operator-lighter

Lighter is defined in the compositing & blending spec as equivalent to porter-duff plus, https://www.w3.org/TR/compositing-1/#porterduffcompositingoperators_plus.

The 'lighter' composite mode already works in <canvas> (via globalCompositeOperation), and it's the same there.

Chrome implemented support in https://bugs.chromium.org/p/chromium/issues/detail?id=439037

Per w3c/svgwg#424 we should expose the new value in webidl and Chrome uses 7 too (https://codereview.chromium.org/779963002/patch/120001/130009)

Differential Revision: https://phabricator.services.mozilla.com/D100605

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1518099
gecko-commit: 12325181f43aee1257922717f8ae4d52e4e502d6
gecko-reviewers: jrmuizel, emilio

svg/types/scripted/SVGAnimatedEnumeration-SVGFECompositeElement.html

@@ -34,13 +34,18 @@
   assert_equals(feCompositeElement.operator.baseVal, SVGFECompositeElement.SVG_FECOMPOSITE_OPERATOR_XOR);
   assert_equals(feCompositeElement.getAttribute('operator'), "xor");
 
+  // Switch to 'lighter'.
+  feCompositeElement.operator.baseVal = SVGFECompositeElement.SVG_FECOMPOSITE_OPERATOR_LIGHTER;
+  assert_equals(feCompositeElement.operator.baseVal, SVGFECompositeElement.SVG_FECOMPOSITE_OPERATOR_LIGHTER);
+  assert_equals(feCompositeElement.getAttribute('operator'), "lighter");
+
   // Switch to 'arithmetic'.
   feCompositeElement.operator.baseVal = SVGFECompositeElement.SVG_FECOMPOSITE_OPERATOR_ARITHMETIC;
   assert_equals(feCompositeElement.operator.baseVal, SVGFECompositeElement.SVG_FECOMPOSITE_OPERATOR_ARITHMETIC);
   assert_equals(feCompositeElement.getAttribute('operator'), "arithmetic");
 
   // Try setting invalid values.
-  assert_throws_js(TypeError, function() { feCompositeElement.operator.baseVal = 7; });
+  assert_throws_js(TypeError, function() { feCompositeElement.operator.baseVal = 8; });
   assert_equals(feCompositeElement.operator.baseVal, SVGFECompositeElement.SVG_FECOMPOSITE_OPERATOR_ARITHMETIC);
   assert_equals(feCompositeElement.getAttribute('operator'), "arithmetic");
 
@@ -56,10 +61,5 @@
   feCompositeElement.operator.baseVal = SVGFECompositeElement.SVG_FECOMPOSITE_OPERATOR_OVER;
   assert_equals(feCompositeElement.operator.baseVal, SVGFECompositeElement.SVG_FECOMPOSITE_OPERATOR_OVER);
   assert_equals(feCompositeElement.getAttribute('operator'), "over");
-
-  // Switch to 'lighter'.
-  assert_equals(SVGFECompositeElement.SVG_FECOMPOSITE_OPERATOR_LIGHTER, undefined);
-  feCompositeElement.setAttribute("operator", "lighter");
-  assert_equals(feCompositeElement.operator.baseVal, SVGFECompositeElement.SVG_FECOMPOSITE_OPERATOR_UNKNOWN);
 });
-</script>
+</script>