... rather than whether it currently has one.

This is for OpenGLSurfaceMode=behind.  In that mode, we need to make the window
transparent wherever a GL-rendered view should not be clipped by GDI-rendered
children or sibling views.

Some apps attach a GL context to the view only temporarily, do some rendering,
and then detach it.  But the GL surface remains, with the rendered graphics.
In order for those to show, the window needs to remain transparent even though
none of its views has a GL context currently attached.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>