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

Meta/CMake/all_the_debug_macros.cmake

@@ -72,6 +72,7 @@ set(HTTPJOB_DEBUG ON)
 set(HUNKS_DEBUG ON)
 set(ICMP_DEBUG ON)
 set(ICO_DEBUG ON)
+set(ILBM_DEBUG ON)
 set(IMAGE_DECODER_DEBUG ON)
 set(IMAGE_LOADER_DEBUG ON)
 set(IMAP_PARSER_DEBUG ON)