Blender Git Loki

UI: Allow changing the active side of line gestures

Line gesture use always the right side of the line as active (the area
of the mesh that is going to be modified) by default.
This adds the ability to change the active side when the line gesture is
active by pressing the F key.
This allows more freedom to position the line after starting the
gestures, as it won't be required to cancel the operation or undo if the
line was used in the wrong direction.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D9301