[osg-users] setGLExtensionFuncPtr priority

[Robert Osfield]

In my testing here on my Kubuntu 18.04 I've found the
glGetProcAddressARB and getGetProcAddress functions were returning non
NULL values for invalid function names. This looks like a driver bug
to me, one that has potentially for affect far more than just the
glGetTextureHandle usage.  dlsym works reliably and has always has
been there as a fallback so I've simply removed the
getGetProcAddressARB code for linux:

https://github.com/openscenegraph/OpenSceneGraph/commit/ec4b618b7aaaa487579b95685d807fdb58aa1e8f

With this the extension set up is now working fine for me, and
osgbindlesstext works again, so I've merged this fix into master, 3.6
and 3.6-TexStorage.

I haven't tested any other OS's yet so would appreciate others testing
out once of these three branches.

Cheers,
Robert