Render only on change. New draw event, * listener.
Added zoom event in VolumeViewer
Added zoom event in SurfaceViewer
Merge pull request #115 from PaulMougel/add-zoom-events

Add zoom events
Panels only draw when updated.
Merged conflicts
Pulled essential color mapping into faster function.
Merge pull request #117 from tsherif/optimize-render-loop

Optimize render loop. Closes #100.