Fix byte swapping in Xext/xvdisp.c

[sunweaver]

We should backport this X.org commit...

commit dfd682b582636a36345144bcf835e3ee46718d90
Author: Alan Coopersmith <alan.coopersmith@sun.com>
Date:   Wed Jan 2 19:27:22 2008 -0800

    X.Org bug 4947/Sun bug 6646626: Xv extension not byte-swapping properly

    X.Org Bugzilla #4947 <https://bugs.freedesktop.org/show_bug.cgi?id=4947>
    Sun bug 6646626 <http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6646626>

    Don't use swapped data after swapping it.   When done swapping data,
    send the swapped data, not the address of the pointer to it, to the client.