incube: Optimize near plane distance estimation to reduce z-fighting

Because DrawTextureCubeEx renders a object with z buffering, choosing
correct near and far clipping plane distances is crucial to minimize
z-fighting.  The object being rendered can have very wide dynamic range,
from few meters to thousands of kilometers, so we must calculate to
obtain clipping plane distances every frame.

Incorporating noise terrain height to this estimation is effective to
reduce z-fighting.