Blender Git Loki

GP Interpolate: Move settings from "gp_sculpt" to a new toolsettings struct - "gp_interpolate"

The "gp_sculpt" settings should be strictly for stroke sculpting, and not abused by
other tools. (Similarly, if other general GP tools need one-off options, those should
go into the normal toolsettings->gpencil_flag)

Furthermore, this paves the way for introducing new settings for controlling the way
that GP interpolation takes place (e.g. with easing equations, or a custom curvemap)