SeekedTo(), since it's only informative to decoders. They
can't modify the seeked frame/time. This also mirrors what
all existing decoders were doing in Seek(). BMediaTrack
is simplified accordingly (resolved two TODOs).
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@38705 a95241bf-73f2-0310-859d-f6bbb57e9c96