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/ImageDecoder.cpp

@@ -9,6 +9,7 @@
 #include <LibGfx/ImageFormats/DDSLoader.h>
 #include <LibGfx/ImageFormats/GIFLoader.h>
 #include <LibGfx/ImageFormats/ICOLoader.h>
+#include <LibGfx/ImageFormats/ILBMLoader.h>
 #include <LibGfx/ImageFormats/ImageDecoder.h>
 #include <LibGfx/ImageFormats/JPEGLoader.h>
 #include <LibGfx/ImageFormats/JPEGXLLoader.h>
@@ -38,6 +39,7 @@ static OwnPtr<ImageDecoderPlugin> probe_and_sniff_for_appropriate_plugin(Readonl
         { PGMImageDecoderPlugin::sniff, PGMImageDecoderPlugin::create },
         { PPMImageDecoderPlugin::sniff, PPMImageDecoderPlugin::create },
         { ICOImageDecoderPlugin::sniff, ICOImageDecoderPlugin::create },
+        { ILBMImageDecoderPlugin::sniff, ILBMImageDecoderPlugin::create },
         { JPEGImageDecoderPlugin::sniff, JPEGImageDecoderPlugin::create },
         { JPEGXLImageDecoderPlugin::sniff, JPEGXLImageDecoderPlugin::create },
         { DDSImageDecoderPlugin::sniff, DDSImageDecoderPlugin::create },