Added support for obsolete attribute

Functions marked with the obsolete attribute will now add [Obsolete] to
the generated output.
Mark untyped overloads as obsolete
AMD_performance_monitors is not const-correct

Workaround fo  bug in rhe specification of AMD_performance_monitors.
Upstream bug report here:
https://www.khronos.org/bugzilla/show_bug.cgi?id=1035
Added workaround for wglMakeCurrent error 6

On some drivers and virtual machines, wglMakeCurrent may fail with a
code 6 when first called. The suggested workaround is to call it in a
loop until it succeeds. See
https://www.opengl.org/discussion_boards/showthread.php/171058-nVidia-wglMakeCurrent()-multiple-threads
Corrected convenience wrappers for Get* and Delete*

GetBoolean, GetInteger6, GetFixedvOES and Delete* are now matched in
the convenience wrapper generator. Methods returning vectors of fixed
size (e.g. 4 ints) are no longer matched.