Decodes an ILBM image structure down to chunks and
({int xsize,int ysize, // 0: size of image drawing area
string bitmapheader, // 2: BMHD chunk
void|string colortable, // 3: opt. colortable chunk (CMAP)
void|string colortable, // 4: opt. colormode chunk (CAMG)
string body, // 5: BODY chunk
mapping more_chunks}) // 6: mapping with other chunks