planetarium/terrain.scm: Produce a properly shaded sample.

planetarium/terrain.scm: Produce a properly shaded sample.

Set the light position in the draw method.  The position is stored in
eye coords and must be updated when the eye moves.  Use a directional
light.  Do not cull back faces.

Fix normal to use the height change (not absolute height!) to east,
west, north and south.  Add more normal tests.

Fix write-sample-terrain to produce the correct number of samples.

Support an optional height->color function.  Change height-color to be
such a function.  Merge draw-colored and draw-shaded into draw-mesh;
conditionally use gl:color-material.

Fix bounds of data plotted.  Since heights adjacent to edges are
imagined flat (not undefined), ALL heights can be used to form quads.