Defines line cap types.
Values:
| IG_VECT_LINECAP_BUTT | The line is squared off at the endpoint of the path. |
| IG_VECT_LINECAP_ROUND | The end of the line becomes a semicircle with center in the end point of the line. |
| IG_VECT_LINECAP_SQUARE | Reserved for future extension. |