[Beignet] Status of cl_khr_gl_sharing

[Rebecca N. Palmer]

As beignet's Debian maintainer, I am considering whether to enable CL-GL 
sharing in our package, given its incomplete state.

Fedora do enable it, and have a Blender crash bug 
https://bugs.freedesktop.org/show_bug.cgi?id=101779 , though as I can't 
reproduce this I can't tell whether disabling it would help.

src/sl_gl_api.c contains clEnqueueAcquire/ReleaseGLObjects which do 
nothing and return success, and are marked "XXX NULL function 
currently."  Is 'nothing' the correct thing for these to do on Intel 
hardware, or is this a potential problem?