OPL: Always cache delays even on non-OPL tracks

This means we don't miss end-of-track dummy events on non-OPL channels.
dro-dosbox: Correctly handle changing tempo values
rdos-raw: Fail if tempo is too slow instead of producing song at wrong speed
Zone 66: Skip unused field and clip loop length fields
synth-pcm: Respect loop end point

Fixes clicking in Zone 66 zim01.670 now the samples are looped correctly.