LibGfx: Add a QOI image format encoder

Previously only a QOI image decoder was available on the system. This
commit adds an encoder as well.

For now it only handles images with 4 channels (RGBA).

Userland/Libraries/LibGfx/QOIWriter.h

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2022, Olivier De Cannière <olivier.decanniere96@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+#include <LibGfx/Bitmap.h>
+
+namespace Gfx {
+
+static constexpr Array<u8, 4> qoi_magic_bytes = { 'q', 'o', 'i', 'f' };
+static constexpr Array<u8, 8> qoi_end_marker = { 0, 0, 0, 0, 0, 0, 0, 1 };
+
+enum class Colorspace {
+    sRGB,
+    Linear,
+};
+
+enum class Channels {
+    RGB,
+    RGBA,
+};
+
+class QOIWriter {
+public:
+    static ByteBuffer encode(Gfx::Bitmap const&);
+
+private:
+    QOIWriter() = default;
+
+    Vector<u8> m_data;
+    void add_header(u32 width, u32 height, Channels, Colorspace);
+    void add_rgb_chunk(u8, u8, u8);
+    void add_rgba_chunk(u8, u8, u8, u8);
+    void add_index_chunk(u32 index);
+    void add_diff_chunk(i8 red_difference, i8 green_difference, i8 blue_difference);
+    void add_luma_chunk(i8 relative_red_difference, i8 green_difference, i8 relative_blue_difference);
+    void add_run_chunk(u32 run_length);
+    void add_end_marker();
+
+    Array<Color, 64> running_array;
+    static u32 pixel_hash_function(Color pixel);
+    void insert_into_running_array(Color pixel);
+};
+
+}