glx: don't create implicit GLXWindow if one already exists

If a GLXMakeCurrent request specifies an X window as its drawable,
__glXGetDrawable will implicitly create a GLXWindow for it. However,
the client may have already explicitly created a GLXWindow for that X
window. If that happens, two __glXDrawableRes resources will be added
to the window.

If the explicitly-created GLXWindow is later destroyed by the client,
DrawableGone will call FreeResourceByType on the X window, but this
will actually free the resource for the implicitly-created GLXWindow,
since that one would be at the head of the list.

Then if the X window is destroyed after that, the resource for the
explicitly-created GLXWindow will be freed. But that GLXWindow was
already destroyed above. This crashes the server when it tries to call
the destroyed GLXWindow's destructor. It also means the
implicitly-created GLXWindow would have been leaked since the
FreeResourceByType call mentioned above skips calling the destructor.

To fix this, if __glXGetDrawable is given an X window, it should check
if there is already a GLXWindow associated with it, and only create an
implicit one if there is not.

Signed-off-by: Erik Kurzinger <ekurzinger@nvidia.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
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