v4l2slvp8dec: Flip the meaning of segment_feature_mode

v4l2slvp8dec: Flip the meaning of segment_feature_mode

In section 9.3.4 a), segment_feature_mode have 0 for absolute and 1 for delta,
while in 19.2, it says the opposite. But the reference code, which usually
rules over the text state that 1 means absolute:

 if (hdr->update_data)
 {
     hdr->abs = bool_get_bit(bool);

And uses it with that meaning to decide weither to override the existing value
or just add the detla. This fixes multiple decoding issues.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1245>