pointgalaxy: Optimize cube background repaint distance

The cube background had checked integral indices vector in the galaxy
volume to see if the viewpoint had moved significantly, but it's not
quite optimal, because it could unnecessarily frequently repaint the
background when you are very close to the integral border.

Now it checks real number distance to determine to repaint only
if really the background has significantly moved.
pointgalaxy: Make rendering parameters customizable in autoexec.cfg

We'd like to migrate old command line mechanism to Squirrel scripts,
but CVars are still much convenient.
pointgalaxy: Make galaxy image file selectable from script

I'd like to use a CVar, but CVar with string doesn't work.
Squirrel function is added to set this value, but it could run twice
for single file name if the application runs in server and client
mode.  I don't care this corner case for the time being because the
file name cannot be changed while the game is running.
pointgalaxy: Make pre-rendered cube background much more seamless

The blobs rotation is calculated consistently regardless of viewpoint's
rotation (only depends on position) in order to make cube background
connect smoothly on edges.  The effect is magnificant that I cannot
tell the cube edges by just looking at the background.

This improvement also reduces apparent difference between direct
rendered scenery and cube background.

This consistent rotation calculation is a bit computationally expensive,
so limited to near blobs when directly rendered.