Playback: resume playback if seeking after song finishes
Playback: loop to song start if lacking any loop target

This makes songs loop forever when requested, even if they don't have a loop
target (they just loop back to the start.)
Playback: add some safety checks to fail if there are any bugs present
cmf-creativelabs: Write song tempo inline

Now we have an initialTempo we don't have to write a placeholder and come back
later to fill in the actual tempo.  Just write it as we go.