Specifies the dropout type.
Values:
| IG_ISIS_DROPOUT_NONE | No dropout. |
| IG_ISIS_DROPOUT_RED | Red dropout. |
| IG_ISIS_DROPOUT_GREEN | Green dropout. |
| IG_ISIS_DROPOUT_BLUE | Blue dropout. |
| IG_ISIS_DROPOUT_CUSTOM | Custom dropout. |
| IG_ISIS_DROPOUT_GREY_CCD | Used in HP Tulip driver. |
| IG_ISIS_DROPOUT_NTSC | Used in HP Tulip driver. |