Per function callbacks

* reenable per function callbacks
* add directCall to omit callbacks when calling OpenGL functions
* fix OpenGL version query
Herb suggests (http://youtu.be/xnqTKD8uD64?t=54m12s) to use const ref, just as in C++98, (unless you are optimizing, and you can back up and justify that optimization with data). Listen to Herb, he makes sense.