decoder: h264: fix long-term reference picture marking process.

decoder: h264: fix long-term reference picture marking process.

Fix reference picture marking process with memory_management_control_op
set to 3 and 6, i.e. assign LongTermFrameIdx to a short-term reference
picture, or the current picture.

This fixes decoding of FRExt_MMCO4_Sony_B.

https://bugs.freedesktop.org/show_bug.cgi?id=64624
https://bugzilla.gnome.org/show_bug.cgi?id=724518

[squashed, edited to use GST_VAAPI_PICTURE_IS_COMPLETE() macro]
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>