Add a render_quality Enum to VolumeRenderer
BUG: Do not update _mapper_types until it is complete
Default render quality is 'default' and rework the volren example UI.
Add a global alpha setting to the volume renderer
Add colormap selection for the volren cut planes.
DRY up the image plane widget iteration.
Merge pull request #23 from enthought/feature/misc-volren-ui-knobs

Misc volume viewer features