Fix AttributeError: 'Win32Platform' object has no attribute 'GLX'

`gl_interop_demo.py` fails on Windows:
```
Traceback (most recent call last):
  File "gl_interop_demo.py", line 37, in initialize
    + get_gl_sharing_context_properties())
  File "X:\Python34\lib\site-packages\pyopencl\tools.py", line 423, in get_gl_sharing_context_properties
    from OpenGL import platform as gl_platform, GLX, WGL
  File "X:\Python34\lib\site-packages\OpenGL\GLX\__init__.py", line 3, in <module>
    from OpenGL.GLX.VERSION.GLX_1_0 import *
  File "X:\Python34\lib\site-packages\OpenGL\GLX\VERSION\GLX_1_0.py", line 14, in <module>
    from OpenGL.raw.GLX.VERSION.GLX_1_0 import *
  File "X:\Python34\lib\site-packages\OpenGL\raw\GLX\VERSION\GLX_1_0.py", line 43, in <module>
    @_p.types(ctypes.POINTER(_cs.XVisualInfo),ctypes.POINTER(_cs.Display),_cs.c_int,ctypes.POINTER(_cs.c_int))
  File "X:\Python34\lib\site-packages\OpenGL\raw\GLX\VERSION\GLX_1_0.py", line 13, in _f
    return _p.createFunction( function,_p.PLATFORM.GLX,'GLX_VERSION_GLX_1_0',error_checker=_errors._error_checker)
AttributeError: 'Win32Platform' object has no attribute 'GLX'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "gl_interop_demo.py", line 81, in <module>
    initialize()
  File "gl_interop_demo.py", line 41, in initialize
    + get_gl_sharing_context_properties(),
  File "X:\Python34\lib\site-packages\pyopencl\tools.py", line 423, in get_gl_sharing_context_properties
    from OpenGL import platform as gl_platform, GLX, WGL
  File "X:\Python34\lib\site-packages\OpenGL\GLX\__init__.py", line 3, in <module>
    from OpenGL.GLX.VERSION.GLX_1_0 import *
  File "X:\Python34\lib\site-packages\OpenGL\GLX\VERSION\GLX_1_0.py", line 14, in <module>
    from OpenGL.raw.GLX.VERSION.GLX_1_0 import *
  File "X:\Python34\lib\site-packages\OpenGL\raw\GLX\VERSION\GLX_1_0.py", line 43, in <module>
    @_p.types(ctypes.POINTER(_cs.XVisualInfo),ctypes.POINTER(_cs.Display),_cs.c_int,ctypes.POINTER(_cs.c_int))
  File "X:\Python34\lib\site-packages\OpenGL\raw\GLX\VERSION\GLX_1_0.py", line 13, in _f
    return _p.createFunction( function,_p.PLATFORM.GLX,'GLX_VERSION_GLX_1_0',error_checker=_errors._error_checker)
AttributeError: 'Win32Platform' object has no attribute 'GLX'
```