Identifies the different formats used for storing image palette in the external file.

Values:

IG_PALETTE_FORMAT_INVALID	Returned when a file could not be read.
IG_PALETTE_FORMAT_RAW_BGR	This is the raw DIB format BGR.
IG_PALETTE_FORMAT_RAW_BGRQ	This is the raw DIB format BGRQ.
IG_PALETTE_FORMAT_RAW_RGB	This is the raw DIB format RGB.
IG_PALETTE_FORMAT_RAW_RGBQ	This is the raw DIB format RGBQ.
IG_PALETTE_FORMAT_TEXT	ASCII text file.
IG_PALETTE_FORMAT_HALO_CUT	Dr Halo .PAL file for use with a CUT file format.