incube: Enable height map texture to affect terrain mesh

Modulated terrain mesh by simplex noise looks good, but we need to
incorporate real terrain info as well for the Earth.  Terrain data is
passed as an equirectangular projection texture just like color map.

Terrain texture is formerly used as a bump map texture, but it now turns
into height map.  We no longer need bump maps since modulated geometry
incorporates its effect.
incube: Introduce base level of noise for better details

Large RoundAstrobjs (such as the Earth) tend to have very little variation
in height compared to their radii.  In such a case, lower octaves in
terrain noise does not matter.  Base height can even be given as a texture,
so we want to supress lower octaves.  Specifying base level achieves it.
incube: Add LOD range parameter to deal with quality and efficiency

TerrainLODRange parameter determines threshold for ratio of distance
between view point and patch in question against patch size.
If a patch has less distance than this threshold, one deeper LOD is
examined.

Setting lower value to this parameter reduces detailed patches and
increases performance.  Setting higher value causes the opposite effect.

This will let us deal with quality and performance tradeoff.  It could be
one global variable, but I think optimal value depends on the other
aspects of RoundAstrobj instances, thus it's a member of instances.

For example, large RoundAstrobjs such as planets tend to require deep
LODs, so setting low value to terrainLODRange helps.