[cairo] OpenGL on Windows

[Brent Fulgham]

Tonight I was able to get an OpenGL-based Cairo test application to build and run under Windows using the 1.12.2 source base.

Attached is the rather large diff to accomplish this.  The majority of the diff consists of the addition of a new "src/win32/GL" directory containing the OpenGL Registry versions of wglext.h and glext.h, which do not ship with Windows. I have generated this change as its own patch (wgl_updates_GLext.patch.zip).

The smaller patch (wgl_updates.patch) contains the actual Cairo source changes.

I found that the default function pointer declarations did not work with the Windows GL implementation because of differences in calling conventions.  I'm not sure if the approach I took to add the declarations is the right one, and was hoping someone could review the change and let me know.

As I write this, I realize that I forgot to update the Makefile to include the GL and WGL source files; I will follow up with this missing piece, once I hear back on the other changes.

Thanks,

-Brent

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