OsdOpenVG Render-on-demand experiment

OsdOpenVG Render-on-demand experiment

An experiment to switch from a timed render loop to rendering
only on demand, in order to increase responsiveness of OSD and
reduce CPU usage.

There could be major negatives to this, therefore all changes
are marked with OSDOVG-ROD-EXPERIMENT for easy greppage. See
notes in source.

There will certainly be temporary breakages.