Rename TexSphere class to RoundAstrobj for better indication

TexSphere was a name based on implementation, but we should name by its
purpose.

Also changed string constant to identify the class in TexSphere.cpp,
which has not matched class name; "TextureSphere" has been assgined for
class "TexSphere".  This issue is fixed by changing both to "RoundAstrobj".

I had a difficult time choosing a better name.  I came up with Other
candidates like CelObj, CelBody, SolidAstrobj and others, but none of them
are sufficiently distinguishable from Astrobj.  Also, we may have nebulae
or particle clouds in the future, so a name which does not associate with
those fuzzy objects is desired.  I settled for RoundAstrobj which imply
mesh rendered, optionally textured and optionally having varying height
terrain.  And most importantly, it's not too long.
cubex: Clear the depth buffer after rendering a CubeEx

DrawTextureCubeEx uses the depth buffer temporarily, but the range of the
depth value is generally different from other rendering passes, so keeping
the values in the buffer is not meaningful.
cubex: Finer detail
surface: Get terrain height from TexSphere if available

The formulation of height determination looks not totally correct.
Could we get proper "height" on a sphere-shaped celestial body?
surface: Modify surface demo to test terrain height acquisition
cubex: Fix inconsistent rotation calculation for DrawTextureCubeEx
cubex: Allow changing LODs by SSD files
surface: Make Tank follow terrain on RoundAstrobj