LibWeb: Parse the shape-image-threshold property

Author: https://github.com/Calme1709
Commit: 9c7202e3f3
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6493
Reviewed-by: https://github.com/tcl3 ✅

Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -3254,6 +3254,7 @@ NonnullRefPtr<StyleValue const> StyleComputer::compute_value_of_property(Propert
     case PropertyID::Opacity:
     case PropertyID::StopOpacity:
     case PropertyID::StrokeOpacity:
+    case PropertyID::ShapeImageThreshold:
         return compute_opacity(absolutized_value);
     case PropertyID::PositionArea:
         return compute_position_area(absolutized_value);