This will allow solvespace to run on pre-OpenGL 3.0 implementations,
such as VirtualBox (and possibly others).
However, VirtualBox's handling of framebuffer objects is buggy;
it tries to draw over our X window and fails to do either.
I hate OpenGL.
Apparently pbuffers are also a deprecated and unportable
way of offscreen rendering. They're not supported by
Chromium OpenGL implementation, which is what 3D-accelerated
VirtualBox uses.
This would also help a future OS X port.