Blender Git Loki

GSoC Editing Grease Pencil Strokes Using Curves

This patch includes all the changes made in the soc-2020-greasepencil-curve branch.

Major changes:

* Add a new "sub-mode" to grease pencil edit mode called "curve edit mode".
* Grease pencil stroke data structure has a pointer to a grease pencil curve. It will be saved with the stroke to the blend file.
* Most edit mode operators have been changed to work in curve edit mode.
* Transformation code has been adjusted to work for grease pencil curves.
* New source file added (gpencil_edit_curve.c) for operators exclusive to curve edit mode.

Differential Revision: https://developer.blender.org/D8660