LibWeb/Painting: Add SaveLayer command

This adds a command for saving the current layer of the canvas.
This is useful for painting content onto a blank background in
isolation and later compositing it onto the canvas.

Libraries/LibWeb/Painting/Command.h

@@ -100,6 +100,7 @@ struct DrawRepeatedImmutableBitmap {
 };
 
 struct Save { };
+struct SaveLayer { };
 struct Restore { };
 
 struct Translate {
@@ -453,6 +454,7 @@ using Command = Variant<
     DrawScaledImmutableBitmap,
     DrawRepeatedImmutableBitmap,
     Save,
+    SaveLayer,
     Restore,
     Translate,
     AddClipRect,

Libraries/LibWeb/Painting/DisplayList.cpp

@@ -98,6 +98,7 @@ void DisplayListPlayer::execute(DisplayList& display_list)
         else HANDLE_COMMAND(DrawRepeatedImmutableBitmap, draw_repeated_immutable_bitmap)
         else HANDLE_COMMAND(AddClipRect, add_clip_rect)
         else HANDLE_COMMAND(Save, save)
+        else HANDLE_COMMAND(SaveLayer, save_layer)
         else HANDLE_COMMAND(Restore, restore)
         else HANDLE_COMMAND(Translate, translate)
         else HANDLE_COMMAND(PushStackingContext, push_stacking_context)

Libraries/LibWeb/Painting/DisplayList.h

@@ -40,6 +40,7 @@ private:
     virtual void draw_scaled_immutable_bitmap(DrawScaledImmutableBitmap const&) = 0;
     virtual void draw_repeated_immutable_bitmap(DrawRepeatedImmutableBitmap const&) = 0;
     virtual void save(Save const&) = 0;
+    virtual void save_layer(SaveLayer const&) = 0;
     virtual void restore(Restore const&) = 0;
     virtual void translate(Translate const&) = 0;
     virtual void add_clip_rect(AddClipRect const&) = 0;

Libraries/LibWeb/Painting/DisplayListPlayerSkia.cpp

@@ -177,6 +177,12 @@ void DisplayListPlayerSkia::save(Save const&)
     canvas.save();
 }
 
+void DisplayListPlayerSkia::save_layer(SaveLayer const&)
+{
+    auto& canvas = surface().canvas();
+    canvas.saveLayer(nullptr, nullptr);
+}
+
 void DisplayListPlayerSkia::restore(Restore const&)
 {
     auto& canvas = surface().canvas();

Libraries/LibWeb/Painting/DisplayListPlayerSkia.h

@@ -28,6 +28,7 @@ private:
     void draw_repeated_immutable_bitmap(DrawRepeatedImmutableBitmap const&) override;
     void add_clip_rect(AddClipRect const&) override;
     void save(Save const&) override;
+    void save_layer(SaveLayer const&) override;
     void restore(Restore const&) override;
     void translate(Translate const&) override;
     void push_stacking_context(PushStackingContext const&) override;

Libraries/LibWeb/Painting/DisplayListRecorder.cpp

@@ -280,6 +280,11 @@ void DisplayListRecorder::save()
     append(Save {});
 }
 
+void DisplayListRecorder::save_layer()
+{
+    append(SaveLayer {});
+}
+
 void DisplayListRecorder::restore()
 {
     append(Restore {});

Libraries/LibWeb/Painting/DisplayListRecorder.h

@@ -114,6 +114,7 @@ public:
     void pop_scroll_frame_id();
 
     void save();
+    void save_layer();
     void restore();
 
     struct PushStackingContextParams {