cairo: Add a cairo graphics device type. And many fixes...

cairo: Add a cairo graphics device type.  And many fixes...

Use the cairo graphics device in the planetarium's mit-snapshot.scm
script to draw the tellurion to a PNG image file without loading Gtk.

Fix the linker commandline(!) and cairo-line-to(!!).  Add cairo-save/
restore around polygon fill options, just because all the rest do.
Punt gtk-graphics/make since a window-less gtk output device is no
longer necessary for planetarium/mit-snapshot.scm.  Clean up some free
variables, other CREF complains, unknown texinfo references, etc.

Rename add-gc-cleanups add-GLIB-cleanups because they are run by the
glib-thread.  Replaced punt-gc-cleanup with execute-glib-cleanup
because you probably should not use the former, rather the latter.