This broke in f67029bd0 and notably bcc1aeeb86.
next_tick needs to accommodate for sub-sample accuracy when
PPQN are not integer samples. e.g. 110 bpm @ 48kHz
PS. Instead of `double` we could use superclock next_tick, and
`one_ppqn_in_superclocks` respectively. This would provide us
with 62 bit significand (instead of 52 bit using double).
Yet for the case of MIDI clock, this will have no real world
effect.
6e2d34df2d