#6 Multiwindow callback functionality(GLUT)

[David O\'Gwynn]

The current incarnation of Opengl.GLUT doens't allow
for multiple window callbacks. To fix this, instead of
using one single global py-callback, I've added a
functionobject and the corresponding c-functions to
reference it so the c-callback will reference the
correct py-callback.

[David O\'Gwynn]

[Tarn Weisner Burton]

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Already implemented in PyOpenGL 2.0