Blender Git Loki

Viewport smoke: add options to draw velocity vectors.

This basically exposes to the UI a function that was only available
through a debug macro ; the purpose is obviously to help debugging
simulations. It adds ways to draw the vectors either as colored needles
or as arrows showing the direction of the vectors. The colors are based
on the magnitude of the underlying vectors.

Reviewers: plasmasolutions, gottfried

Differential Revision: https://developer.blender.org/D1733