[jitter] Bug report: jit.gl.slab not able to change context

I don't think this problem has anything to do with shader files but
instead with the offscreen context generated by jit.gl.texture
internal to jit.gl.slab for processing input textures.  Clearly
something is not being initialized or destructed appropriately and is
throwing errors to the Max window.  I've noticed this behavior as
well.  If anyone has any simple patches that generate these errors
consistently, please post 'em.

thanks,
wes