Auto merge of #19436 - aethanyc:shape-image-threshold, r=heycam

style: Support 'shape-image-threshold' property

This has been reviewed in https://bugzilla.mozilla.org/show_bug.cgi?id=1265343

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components/style/properties/longhand/box.mako.rs

@@ -911,6 +911,14 @@ ${helpers.single_keyword("-moz-orient",
     }
 </%helpers:longhand>
 
+${helpers.predefined_type(
+    "shape-image-threshold", "Opacity", "0.0",
+    products="gecko",
+    gecko_pref="layout.css.shape-outside.enabled",
+    animation_value_type="ComputedValue",
+    spec="https://drafts.csswg.org/css-shapes/#shape-image-threshold-property",
+)}
+
 ${helpers.predefined_type(
     "shape-outside",
     "basic_shape::FloatAreaShape",