GTK3 plugin: Support OpenGL/WGL on Windows

Hi,

This attempts to implement the gtkglsink element on Windows using WGL, as there were some more gotchas that are along the way, since we need to juggle with libepoxy along the way.

The thing left is that the shaders seem not to be attached to the GstGLContext that we create when exiting from the gtkglsink. Need to find a way to reattach those--did I happen to miss something here?

I had gtk version checking at 3.24.38the upcoming 3.24.43 as this will depend on an MR in GTK as well in GDK-Win32.

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Edit: finally got this working, crash free, on Windows!

With blessings, thank you!