cdgparse: improve seeking
cdgparse
is marking CDG_CMD_MEMORY_PRESET
frames as key frames, so seeking using GST_SEEK_FLAG_KEY_UNIT
jump to one of those, preventing visual artifacts.
We could improve the non key-unit seeking by jumping backward to the nearest keyframe and redecode the frames between the kf and the seeking point. Decoding cdg is really fast so that shouldn't be an issue.