Measurement units.
Values:
| IG_VECT_MUNIT_UNKNOWN | Unknown. |
| IG_VECT_MUNIT_MM | Metrics units - millimeter. |
| IG_VECT_MUNIT_CM | Metrics units - centimeter. |
| IG_VECT_MUNIT_M | Metrics units - meter. |
| IG_VECT_MUNIT_INCHES | English units - inches. |
| IG_VECT_MUNIT_PLTS | Plot units - 1/40 mm. |
| IG_VECT_MUNIT_TWIPS | Plot units - 1/1440 inch. |