Add OpenGL Support for the GDK Windows Backend

This adds support for OpenGL to the GDK Windows backend using the WGL API
calls, which enables programs that uses the GTK+ GLArea widgets to work on
Windows as well.

This also adds a simple utility function to query for the version of OpenGL
that is supported by the Windows system, like the one provided by the X11
backend.

Many thanks to Alex (and Emmanuele, who started the OpenGL integration in
GTK+) who offered advice and help along the way, as well as the X11 and
Wayland backend for this work to refer to and to model upon.

https://bugzilla.gnome.org/show_bug.cgi?id=740795

gdk/win32/gdkwin32.h

@@ -35,6 +35,7 @@
 #include <gdk/win32/gdkwin32screen.h>
 #include <gdk/win32/gdkwin32window.h>
 #include <gdk/win32/gdkwin32misc.h>
+#include <gdk/win32/gdkwin32glcontext.h>
 
 #undef __GDKWIN32_H_INSIDE__