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.
Travis OS X builders randomly break in ways that aren't reproducible
outside of their VMs, have no Google hits, and are very aggravating;
the two common ones are:
* hdiutil: create failed - Resource busy
* make[2]: write error
This commit drags SNR back into a tolerable range with a gross hack.
whitequark