It was never really needed, since both Linux and OS X, where
GlOffscreen is used, guarantee that the API we need is present,
on all OS versions we're interested in.
Also, reorganize GlOffscreen consistently with the rest of our
codebase, and don't use RAII for OpenGL resource management because
of its requirement for an active context.
After this change, SolveSpace does not contain nonfree assets.
Additionally, Perl is not required for the build.
Note that in the US, case law suggests that copyright does
not apply to bitmap fonts:
http://www.renpy.org/wiki/renpy/misc/Bitmap_Fonts_and_Copyright
Nevertheless, it was prudent to replace the asset with something
that is unambiguously free.
