LibGfx/PortableFormat: Add PortableFormatWriter

Currently, the encoder only output ppm files, but it can easily be
extended to support both PBM and PGM.

Userland/Libraries/LibGfx/PortableFormatWriter.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2023, Lucas Chollet <lucas.chollet@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/ByteBuffer.h>
+#include <LibGfx/Bitmap.h>
+
+namespace Gfx {
+
+// This is not a nested struct to work around https://llvm.org/PR36684
+struct PortableFormatWriterOptions {
+    enum class Format {
+        ASCII,
+        Raw,
+    };
+
+    Format format = Format::Raw;
+    StringView comment = "Generated with SerenityOS - LibGfx."sv;
+};
+
+class PortableFormatWriter {
+public:
+    using Options = PortableFormatWriterOptions;
+
+    static ErrorOr<ByteBuffer> encode(Bitmap const&, Options options = Options {});
+
+private:
+    PortableFormatWriter() = delete;
+
+    static ErrorOr<void> add_header(ByteBuffer&, Options const& options, u32 width, u32 height, u32 max_value);
+    static ErrorOr<void> add_pixels(ByteBuffer&, Options const& options, Bitmap const&);
+};
+
+}