mpegvideoparse: Seeks to I frames instead of the sequence header right before the I frame

Submitted by Sebastian Dröge @slomo

Link to original bug (#674238)

Description

Currently mpegvideoparse seeks to I frames instead of the sequence header right before the I frame, which usually comes directly before the I frame. As the sequence header could change for every I frame, the parser should use this as key units.

The current behaviour requires caching of the sequence header inside decoders and assuming that it didn't change. As a result, when this caching does not happen, accurate seeking is impossible because the decoder has to skip over until the next sequence header in front of the next I frame.