Add threshold parameter to FindBrightestAstrobj

Since FindBrightestAstrobj became capable to return multiple light
sources, it may include too faint stars to the returned list.
Sometimes it may be useful, but we want to omit evaluating such
faintest light sources when rendering every frame.  So we add a
way to truncate faint stars by providing threshold.  Note that this
threshold is different from eclipseThreshold, which is used to
truncate eclipse checking calculation.
Change CoordSys::name's obsolete char-pointer type to dstring

Types of CoordSys::fullname and member of CoordSys::extranames are
also changed.  They are oldest data structures of the whole project
since the time when dstring type was not mature, but manually managing
heap memory for those strings is tough and error prone task.
We do not want to keep them forever.  Let's migrate eariler and test
longer.
Add StellarError class to handle exceptions in CoordSys::readFile()

Traditionally, we have been using return codes to determine if any
error happened, but it doesn't scale well.  We can handle the errors
in more C++-like way.
Add CoordSys::sqcalc() to interpret real values in SSD as Squirrel

We have been using calc3(), but it's so obsolete since we have a
script language framework.  Let's pass it to Squirrel interpreter.
Add albedo property to TexSphere

It currently does nothing.
Make TexSpheres reflect lights from other light sources

A TexSphere instance's absmag is derived from light sources
illuminating the area of the TexSphere's location.  The light sources
include other TexSpheres being illuminated, so multiple reflections
are also simulated, although the reflections are calculated only once
in a simulation step.  It means secondary and tertially reflections
delay several frames, but it wouldn't be noticeable.

The albedo property introduced in the previous commit is taken into
account for calculating intensity.

This reflection update costs O(n^2) of intensity evaluation with n
being the number of all TexSpheres in the universe, so it may be
suboptimal to calculate every frame.  We may think some mechanism
to cull CoordSys trees with low brightness.
Moon's fragment shader accept multiple light sources

We have been supporting multiple lights for flat planets, so we can
easily port the functionality to Moon's shader.

Also supported light source colors to simulate earthshine.
It looks cool!