Blender Git Loki

Curved strokes for paint modes.

This commmit introduves a first rough implementation of strokes guided
by screen space curves for artist review. This is somewhat limited
because it does not allow panning (which in 3d viewport wouldn't help
anyway).

To use select Curve under the stroke menu, or press the A key
By dragging, the user can define the first bezier control points. It is
also possible to select and edit the bezier handles independently. To
constrain the handles to follow each other (similar to aligned bezier
curves), use shift before selecting and dragging a handle. To avoid
accidentally adding a curve point, the user needs to explicitly use
Ctrl-Click to add another bezier point. To confirm the stroke use either
enter or spacebar.

TODOs: No pressure simulation or dynamic curve subdivision is done in
this first implementation. Also the draw calls for the bezier could be
optimized better.