LibGfx: Add IPC serialization for ShapeFeature

Font transport needs shape feature tags and values to cross the
compositor IPC boundary with their owning LibGfx representation. Add a
ShapeFeature specialization in its own source file and encode the fixed
tag bytes together with the feature value.

This is preparatory work required to add IPC between the main and
compositor threads.

Libraries/LibGfx/CMakeLists.txt

@@ -53,6 +53,7 @@ set(SOURCES
     SharedImage.cpp
     SharedImageBuffer.cpp
     ShareableBitmap.cpp
+    ShapeFeature.cpp
     Size.cpp
     SkiaBackendContext.cpp
     SkiaUtils.cpp

Libraries/LibGfx/ShapeFeature.cpp

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2026-present, the Ladybird developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGfx/ShapeFeature.h>
+#include <LibIPC/Decoder.h>
+#include <LibIPC/Encoder.h>
+
+namespace IPC {
+
+template<>
+ErrorOr<void> encode(Encoder& encoder, Gfx::ShapeFeature const& feature)
+{
+    TRY(encoder.append(reinterpret_cast<u8 const*>(feature.tag), sizeof(feature.tag)));
+    TRY(encoder.encode(feature.value));
+    return {};
+}
+
+template<>
+ErrorOr<Gfx::ShapeFeature> decode(Decoder& decoder)
+{
+    Gfx::ShapeFeature feature {};
+    TRY(decoder.decode_into(Bytes { reinterpret_cast<u8*>(feature.tag), sizeof(feature.tag) }));
+    feature.value = TRY(decoder.decode<u32>());
+    return feature;
+}
+
+}

Libraries/LibGfx/ShapeFeature.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Vector.h>
+#include <LibIPC/Forward.h>
 
 namespace Gfx {
 
@@ -22,6 +23,16 @@ using ShapeFeatures = Vector<ShapeFeature, 4>;
 
 }
 
+namespace IPC {
+
+template<>
+ErrorOr<void> encode(Encoder&, Gfx::ShapeFeature const&);
+
+template<>
+ErrorOr<Gfx::ShapeFeature> decode(Decoder&);
+
+}
+
 namespace AK {
 
 template<>