H264: hotfix for speedloss on frame threading and h264 files with slices.

This fix is not ideal as it still limits the multithreading on field pictures
to the 2nd field only.
Ill try to fix it properly to allow both fields to decode concurrently but this
needs more work.

This bug exists since and was caused by:
commit ea6331f8bbaf3343faec6ffe77a6218a016a3af5
Author: Ronald S. Bultje <rsbultje@gmail.com>
Date:   Mon Jun 20 10:24:33 2011 -0400

    h264-mt: fix deadlock in packets with multiple slices (e.g. MP4).
