[GRASS5] NVIZ updates

[Helena]

I have added 2 modfications from Tomas to nviz volume support:

1. Default polygon resolution for drawing isosurfaces is now a smaller number
(higher resolution) to avoid situation when nothing is drawn.

2. Load state/Save state was added for volume objects.

Bob, after the nviz startup has changed, Load state/Save state does not work
- it gives an Error: can't read env(GISDBASE)
Can this be fixed before the release or should I file a bug report?

Let me know if there are any problems with the new additions,

Helena