Expose WGL context to applications

This exposes the WGL HGLRC context handle to applications.

This will enable GStreamer, for example, to be able to create a GL
context that shares resources with Cogl's context.

https://bugzilla.gnome.org/show_bug.cgi?id=724992
(cherry picked from commit 76e79aa09dfed3cea9aaafbe43b4648f82f1aaa8)