LibGfx/ILBM: Add an IFF-ILBM decoder :)

IFF was a generic container fileformat that was popular on the Amiga
since it was the only file format supported by Deluxe Paint.

ILBM is an image format popular in the late eighties/nineties
that uses the IFF container.

This is a very first version of the decoder that only supports
(byterun) compressed files with bpp <= 8.

Only the minimal chunks are decoded: CMAP, BODY, BMHD.

I am planning to add support for the following variants:

- EHB (32 colours + lighter 32 colours)
- HAM6 / HAM8 (special mode that allowed to display the whole Amiga
4096 colours / 262 144 colours palette)
- TrueColor (24bit)

Things that could be fun to do:

- Still images could be animated using color cycle information

Userland/Libraries/LibGfx/ImageFormats/ILBMLoader.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2023, Nicolas Ramz <nicolas.ramz@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGfx/ImageFormats/ImageDecoder.h>
+
+namespace Gfx {
+
+struct ILBMLoadingContext;
+
+class ILBMImageDecoderPlugin final : public ImageDecoderPlugin {
+public:
+    static bool sniff(ReadonlyBytes);
+    static ErrorOr<NonnullOwnPtr<ImageDecoderPlugin>> create(ReadonlyBytes);
+
+    virtual ~ILBMImageDecoderPlugin() override;
+
+    virtual IntSize size() override;
+    virtual ErrorOr<Optional<ReadonlyBytes>> icc_data() override;
+
+    virtual ErrorOr<ImageFrameDescriptor> frame(size_t index, Optional<IntSize> ideal_size = {}) override;
+
+private:
+    ILBMImageDecoderPlugin(ReadonlyBytes, NonnullOwnPtr<ILBMLoadingContext>);
+    NonnullOwnPtr<ILBMLoadingContext> m_context;
+};
+
+}