Change for Windows only: Make demos and examples build again.

Move the inclusion of windows.h from glext.h to gl.h.  There are a few
good reasons for this:
- Including it in gl.h is more strightforward and makes it easier to
troubleshoot include file problems.  It was sometimes hard to find it
in glext.h.
- Needed to define APIENTRY correctly.  APIENTRY is used in gl.h.
- glut.h needs windows.h but does not include it, expecting the app to
include it.  Including it in gl.h if not already included by the app
makes glut.h happy.  This allows one to write a gl/glut app without
explictly including windows.h, which may only be really important for
toy apps like our samples and demos.  But a "real" app can still
include windows.h if it wants.

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