player: add seek_fast config option

Add a seek_fast player config option as an alternative to currently available normal and accurate options.

The new seek_fast option restores playback from next keyframe from requested seek position and takes precedence over seek_accurate when both options are enabled. Depending on how keyframes are distributed in the video, this can be even over 20x faster than standard seeking on some files (at the expense of being more inaccurate).