Unfortunately there are plenty of other places left. Perhaps we should
consider creating our own window when the context becomes invalid and making
the context current on that instead.

Updating a texture while it is attached to the currently bound FBO is
something GL implementations tend to get wrong. NVIDIA fails at
glTexSubImage2D(), fglrx and Mesa with glTexImage2D(). I'm afraid to try what
happens on OS X. Fortunately we never use glTexImage2D() while a texture is
attached to an FBO, so we only need to care about glTexSubImage2D().

What happens is that we get the WM_WINE_SETACTIVEWINDOW message sent by the
other thread after we already changed the foreground window back to our own
window.

This is needed so that ImagingFactory can have a threading model of Both.