Do not throw error when a CoordSys object in Squirrel VM become NULL

Now that CoordSys object tree is fairly dynamic, we cannot assume that
these objects are stationary.  Specifically, symbolic bookmarks can
easily become broken.  The callers of CoordSys::sq_refobj() should take
care of returned NULLs appropriately.
Make extent, isolated and cs_radius properties accessible from Squirrel
Make materialized star systems have enough cs_radius as a solar system

This will make the solar system browser work fine with those systems.
Expose fontwidth() API to Squirrel scripts
Add column headers to warp destination selection window

Clicking on a header makes the list be sorted by the column value.
Clicking the same column again reverses the order.

We would also like to change column order and widths, but it's not
implemented yet.
Fix random star labels fail to display on correct position seen from Sol

We have not properly calculated transformation matrix from the universe
to the local (viewer's) CoordSys.  This issue has not been exposed since
it looks fine when we're viewing from a CoordSys having the same
orientation as the universe, but it's obvious when we're in Sol.
Non-random stars have not had this problem.
Add GLwindow's onMouseState event handler for Squirrel scripts

Also changed definition of GLwindowSizeable::mouseCursorState to allow
setting onMouseState event handler without interfering with window
border changing.
Allow sizing column widths in warp destination window

Also added visited column to show the effect.
Fix a bug that velocity gets screwed when entering materialized star

We observe a phenomenon that the warp velocity vector suddenly goes to
unexpected direction when we approach a randomly generated star which
is just materialized.  This was caused by referring to old position vector
in warp calculation method, which should be updated when the destination
CoordSys is altered in materializeStar().

We are also observing strange velocity behavior when warping to Sol,
but it seems due to another cause.
Fix space_debug.ssd had invalid orbit specification

Also made OrbitCS::readFile() report a warning if it could not find a
orbit with given name in the file.  This would make debug easy.
Fix star name for Rigil Kentaurus in space_debug.ssd
Change type of shader file name strings to gltestp::dstring

We should make all dynamic strings gltestp::dstring instead of
cpplib::dstring or std::string, since we need to export these string
types to extension DLLs.

Also added a typedef for "st" and shorthand type for std::vector<String>.
Add GLSL file name as TexSphere properties accessible from Squirrel
Add shaders for flat textured planets in randomly generated systems

Now they take effect of HDR exposure.
galaxy: Temporarily revive rendering logic for the Galaxy

The codes are commented out since the previous project, gltest, is
under development.  They are so old that written in C.  We had to
modify types to let they compile at all.
pointgalaxy: Experiment with point sprite rendering for galaxy
Fix embarrassing spelling in draw_gs
pointgalaxy: Give up using point sprites; use billboards instead

The point sprites feature of OpenGL is too device-dependent to use.
We fell back to good old billboards with random rotation to render
a blob in the Galaxy.
pointgalaxy: Cache blobs to enhance rendering speed
pointgalaxy: Import an artificial galaxy map image

We cannot see Milky Way galaxy from outside, so it's all art.