avvidec: Ensure skipping strategy gets properly reset

avvidec: Ensure skipping strategy gets properly reset

When switching playback modes, like from TRICKMODE or TRICKMODE_KEY_UNITS
back to regular playback, we need to make sure we set the skip mode
back to the default setting.

While this field would be properly reset when we *have* feedback from
downstream (i.e. diff != G_MAXINT64), it would not be reset during
the initial phase (i.e. when the decoder hasn't pushed a buffer yet,
and therefore the sink hasn't sent back QoS information).

This avoids dropping plenty of frames when going back to regular playback