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

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