Investigate ways to change the lighting in Vispy

[astrofrog]

For the isosurface with the VolumeVisual, I think this is done in here:

https://github.com/vispy/vispy/blob/master/vispy/visuals/volume.py#L115

For the IsoSurfaceVisual, I think it's done here:

https://github.com/vispy/vispy/blob/master/vispy/visuals/mesh.py#L81

@PennyQ - I'm assigning you to this issue as discussed yesterday on the hangout.

[PennyQ]

Just found the lighting in VolumeVisual is still in vispy's todo lists, as in https://github.com/vispy/vispy/blob/master/vispy/visuals/volume.py#L45

and current lighting is a hardcoded one https://github.com/vispy/vispy/blob/master/vispy/visuals/volume.py#L92