tdf#105162: Need to call epoxy_handle_external_wglMakeCurrent()

Somewhere, in time before calling epoxy-wrapper OpenGL functions. This
seems to be a good place. Educated guess based on debugging, turned
out to work.

Change-Id: Id3f1dfb0c5a9dbfb166c47ea81db902af1a4fc08

vcl/opengl/win/gdiimpl.cxx

@@ -96,6 +96,8 @@ void WinOpenGLContext::makeCurrent()
 
     clearCurrent();
 
+    epoxy_handle_external_wglMakeCurrent();
+
     if (!wglMakeCurrent(m_aGLWin.hDC, m_aGLWin.hRC))
     {
         g_bAnyCurrent = false;