Blender Git Loki

GPencil UI: Experimental "tool palette" pie menu

Double-tapping D brings up a pie menu providing access to many of the different
Grease Pencil drawing and editing tools.

The current implementation is a quick test of how this could work.
* Double-tapping D is the best compromise I managed to find so far which allows
the pie to show, but without getting triggered everytime something else happens
* I've left off one or two tools since they didn't work terribly well,
as they rely on mouse position at time of invocation
* Even now, the drawing tools take a little getting used to (i.e. you must immediately
start drawing once you click. But, once gotten used to, it seems to be ok...