[AD] polygon3d patch - and small mixer patch

["Ben Davis" <ben@xxxxxxxxxx>]

I realised that silent voices don't ever have volume ramps in progress -
hence the small mixer patch.

Here is also a patch for poly3d.c, aintern.h, test.c, ex3d.c, allegro._tx
and thanks._tx (was I supposed to modify that myself?). Can someone check
that the documentation is acceptable, and then commit the patch please?

Thanks,

Ben Davis

Attachment: poly3d.diff
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Attachment: mixer.c.diff
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