add psuedo-uniform changes for other UI controls for HUD
add psuedo-uniform changes for other UI controls for HUD

The HUD instruments the control of uniforms in volume_renderer, to
track a set of most recent changes that expires by timer.  For other
UI controls not reified as uniforms, insert psuedo change events into
the same tracking structure so that the HUD renderer can display them
consistently.

informatics-isi-edu/volspy