Windows: Don't release the OpenGL context.

The release method was added to prevent lockups when the GL context is
accessed from another thread but there should now be no circumstance
under which any other thread makes the UI OpenGL context current (on
windows at least). Not releasing makes OpenGL video rendering work again
on windows when using OpenGL for the UI.

Refs #9268

mythtv/libs/libmythui/mythrender_opengl.cpp

@@ -102,8 +102,10 @@ void MythRenderOpenGL::doneCurrent()
 
 void MythRenderOpenGL::Release(void)
 {
+#if !defined(Q_WS_WIN)
     while (m_lock_level > 0)
         doneCurrent();
+#endif
 }
 
 void MythRenderOpenGL::MoveResizeWindow(const QRect &rect)