Though this patch to fix ticket #6668, I belive it
is unnecessary to set SLICE_FLAG_ALLOW_FIELD flag to other
hwaccels(dxva, vdpau, etc). Please also refer the orginal comment
of 9cb150c9

Should also fix ticket #8442.
Signed-off-by: Zhong Li <zhong.li@intel.com>
Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>