Add possibility to determine min altitude of volume shapes

[ComBatVision]

It is now not possible to render path, polygon or ellipse with extrude which ends not on the terrain, but on some specified altitude.

This task can be done in two ways:

1. Specify constant min altitude for all points
2. Specify thickness of volume which will calculate altitude offset based on each point altitude.

Second way is more flexible, but wee need to investigate what is more standard way in other GIS systems.