Implement support for the drop-shadow filter (#30439)

* Implement support for `drop-shadow`

* Clean up remnant from early attempts

* Fix misleading comments on GenericSimpleShadow
If Servo-specific `style` changes will need to be upstreamed anyway, I might as well fix a thing that had thrown me off!

* Revert "Fix misleading comments on GenericSimpleShadow"

This reverts commit cdc810b826ac082041adc212c24649ee3b86ca0a.

* Clean up an import

* Update test expectations

* Fix missing expectation on Layout 2013

components/layout/display_list/builder.rs

@@ -62,7 +62,7 @@ use crate::display_list::items::{
     PushTextShadowDisplayItem, StackingContext, StackingContextType, StickyFrameData,
     TextOrientation, WebRenderImageInfo,
 };
-use crate::display_list::{border, gradient, ToLayout};
+use crate::display_list::{border, gradient, FilterToLayout, ToLayout};
 use crate::flow::{BaseFlow, Flow, FlowFlags};
 use crate::flow_ref::FlowRef;
 use crate::fragment::{
@@ -1975,8 +1975,14 @@ impl Fragment {
             .translate(-border_box_offset.to_vector());
 
         // Create the filter pipeline.
+        let current_color = self.style().clone_color();
         let effects = self.style().get_effects();
-        let mut filters: Vec<FilterOp> = effects.filter.0.iter().map(ToLayout::to_layout).collect();
+        let mut filters: Vec<FilterOp> = effects
+            .filter
+            .0
+            .iter()
+            .map(|filter| FilterToLayout::to_layout(filter, &current_color))
+            .collect();
         if effects.opacity != 1.0 {
             filters.push(FilterOp::Opacity(effects.opacity.into(), effects.opacity));
         }

components/layout/display_list/conversions.rs

@@ -17,6 +17,11 @@ pub trait ToLayout {
     fn to_layout(&self) -> Self::Type;
 }
 
+pub trait FilterToLayout {
+    type Type;
+    fn to_layout(&self, current_color: &RGBA) -> Self::Type;
+}
+
 impl ToLayout for BorderStyle {
     type Type = wr::BorderStyle;
     fn to_layout(&self) -> Self::Type {
@@ -35,9 +40,9 @@ impl ToLayout for BorderStyle {
     }
 }
 
-impl ToLayout for Filter {
+impl FilterToLayout for Filter {
     type Type = wr::FilterOp;
-    fn to_layout(&self) -> Self::Type {
+    fn to_layout(&self, current_color: &RGBA) -> Self::Type {
         match *self {
             Filter::Blur(radius) => wr::FilterOp::Blur(radius.px(), radius.px()),
             Filter::Brightness(amount) => wr::FilterOp::Brightness(amount.0),
@@ -48,8 +53,14 @@ impl ToLayout for Filter {
             Filter::Opacity(amount) => wr::FilterOp::Opacity(amount.0.into(), amount.0),
             Filter::Saturate(amount) => wr::FilterOp::Saturate(amount.0),
             Filter::Sepia(amount) => wr::FilterOp::Sepia(amount.0),
-            // Statically check that DropShadow is impossible.
-            Filter::DropShadow(ref shadow) => match *shadow {},
+            Filter::DropShadow(ref shadow) => wr::FilterOp::DropShadow(wr::Shadow {
+                blur_radius: shadow.blur.px(),
+                offset: wr::units::LayoutVector2D::new(
+                    shadow.horizontal.px(),
+                    shadow.vertical.px(),
+                ),
+                color: shadow.color.clone().into_rgba(*current_color).to_layout(),
+            }),
             // Statically check that Url is impossible.
             Filter::Url(ref url) => match *url {},
         }

components/layout/display_list/mod.rs

@@ -6,7 +6,7 @@ pub use self::builder::{
     BorderPaintingMode, DisplayListBuildState, IndexableText, StackingContextCollectionFlags,
     StackingContextCollectionState,
 };
-pub use self::conversions::ToLayout;
+pub use self::conversions::{FilterToLayout, ToLayout};
 
 mod background;
 mod border;