vp8: fix check for disabling the loop filter (again).
vp8: fix check for disabling the loop filter (again).

Improve condition to disable the loop filter. The previous heuristic
used to check all filter levels, for all segments. It turns out that
only the base filter_level value defined in the frame header needs
to be checked.

This fixes 00-comprehensive-013.

ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi