GL_INTEL_performance_queries is missing.
@0ad is currently[ trying to migrate](https://code.wildfiregames.com/D721) from custom made headers to glad2, thanks a lot for this opportunity. It seems however that Glad2 is missing an extension called `GL_INTEL_performance_queries` not to be confused with `GL_INTEL_performance_query` although adding the last one as an extension allows us to compile with no problem.

The only documentation that seems to exist about it seems to have been written by one of our former devs here: http://zaynar.co.uk/docs/gl-intel-performance-queries.html

Is it of any interest to you to add it?
