khronos-update.py: add script to simplify update of Khronos headers & xml files
The idea is to have the canonical source of each of those files
available without having to remember anything, and to be able to update
all the Vulkan files by simply running bin/khronos-update.py vulkan.
The script also handles the fact all the EGL/GL/GLES* headers depend on
the KHR header, and the former should not be updated without updating
the latter.
Note: I wasn't able to find the canonical source for GL/gl.h, GL/glx.h and GLES3/gl3ext.h.
Is Mesa the canonical source? If so, I'll update them to None.