layout: Add support for mix-blend-mode: plus-lighter (#34057)

This just requires translating the style value into a WebRender value.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout/display_list/conversions.rs

@@ -102,6 +102,7 @@ impl ToLayout for MixBlendMode {
             MixBlendMode::Saturation => wr::MixBlendMode::Saturation,
             MixBlendMode::Color => wr::MixBlendMode::Color,
             MixBlendMode::Luminosity => wr::MixBlendMode::Luminosity,
+            MixBlendMode::PlusLighter => wr::MixBlendMode::PlusLighter,
         }
     }
 }