Blender Git Loki

Curves: Curve Pen

This tool can be used to rapidly edit curves. It follows the design proposed here with the inclusion of a few changes. The final set of functionalities for Bezier splines are as follows:

* Add point: Click on empty space to add a new point connected to the currently selected point(s). If no point is currently selected, a new spline is created.
* Only clicking adds a point with handles of type `vector`.
* Click and drag to move handles as `auto` type handles.
* Press Shift while dragging to set handles as `free` type handles.
* Clicking two endpoints consecutively will close the spline to form a cycle.
* Make cut: Hold Ctrl and click to insert a vertex into the spline at the clicked location.
* Move control points and handles: Click and drag to move control points or handles.
* Alter segment: Click and drag on a curve segment to alter its shape.

All the above functionalities except for alter segment works similarly in the case of Poly and NURBS splines, minus the handle interactions.

Differential Revision: https://developer.blender.org/D12155