Close #4883

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

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Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

They are still enumerated many times though...

It had no effects since both enumeration members had the same value.

It is not reliable if the aout is reused, and the text support seems
incomplete in goom.

This reverts commit 48e39d44.

 It is needed to apply the patch allowing to flush the packetizer/decoders
without having to change the API for now.

It was rather confusing that XPM, Xinerama and Xext seemed absolutely
required even though only skins2 depends on them. Also the check for
libtar is now skipped if skins2 are disabled.