This adds support for line gesture to SculptGestureContext and
implements a Mask Line tool, which affects everything to the right of a plane
defined by the straightline gesture.
For this to work, a new WM_gesture_straightline_oneshot_modal is needed
which only runs exec when the gesture is over.
Added as experimental as it does not have icon.
Reviewed By: Severin
Differential Revision: https://developer.blender.org/D8722
8c81b3fb8b