Switch submodule to https://github.com/capnm/Khronos_XML_API

The repository `https://github.com/capnm/Khronos_XML_API` seems more often updated than the one we were using before.

However, the new XML file "introduces" a new syntax for parameters, which is `<param><ptype>GLuint</ptype> <name>baseAndCount</name>[2]</param>`.
Only the `glPathGlyphIndexRangeNV` extension uses it right now, so I don't know whether it's really a new syntax or just a bug in the specs.

This PR would also add support for OpenGL 4.5
Fix #89