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. |