Minor fixes and changes

I have made the following changes:
 - LOD didn't seem to work properly - it would never show the last LOD level (and not work at all if there are only two LOD levels obviously).
 - Parallel projection still had a near clip plane. Orthographic projection can have a negative near clipping plane, since the vertices will be projected into same position on screen regardless of their distance to camera. This makes it possible to zoom in without clipping.
 - Added a way to change factor by which GLC_Viewport::reframe() estimates size of the scene - in my case a lower value is more comfortable. Also now reframe can just re-center view without changing the zoom value.

LOD problem: previously if mesh had `n` LOD levels, then the lowest LOD level `n-1` will start from range:
`(int)(range / 100.0) * (n-1) = n-1`
`(int)(range / 100.0) = 1`
That is, for any range equal to 100.0 or higher. In your code 100.0 means the object is zero pixels in size on screen, making the last LOD level useless.