The previous code only used the 1st multiplicand was use to
determine the direction of rounding, breaking commutative property
`muldiv_round (1, 3, 4) != muldiv_round (3, 1, 4)`
This fixes various rounding issues. Notably superclock to sample
conversion must always round down when playing forward.
`::process (start, end, speed = 1)` uses exclusive end.
Processing begins at `start` and end ends just before `end`.
Next cycle will begin with the current end.
One example where this failed:
- New session at 48kHz
- Change tempo to 130 BPM
- Enable snap to 1/8 note
- Snap playhead to 1|3|0
- Enable Metronome
- Play
`assert (superclock_to_samples ((*i).sclock(), sample_rate()) < end);`
end = 177231 samples == superclock 1042118280
A grid point is found at superclock 1042116920 (that is < 1042118280).
However converting it back to samples rounded it to sample 177231 == end,
while actual location is 1360 super-clock ticks before end.
The metronome click has to be started this cycle, since the same
position will not be found at the beginning of the next cycle, with
start = 177232.
Similarly a samplecnt_t t, converted to music-time and back must not be
later than the given sample.
```
timepos_t tsc (t);
assert (timepos_t::from_ticks (tsc.ticks ()).samples () <= t);
```
IOW. When playing forward, all super-clock time between 1|1|0 and 1|1|1
should round down to 1|1|0. "We have not yet reached the first tick".
When switching backends, the effective sample-rate is zero.
This only affects the butler thread (the only active thread when
stopped). The actual issue here is the butler calling
"non-realtime-stop" without a backend. However fixing 0/0
generally seems appropriate.
```
#0 in int_div_round<long>(long, long) (x=0, y=0) at ../libs/pbd/pbd/integer_division.h:36
#1 in Temporal::samples_to_superclock(int64_t, int) (samples=0, sr=0) at ../libs/temporal/temporal/superclock.h:39
#2 in Temporal::timepos_t::timepos_t(long) (this=0x7f94bc0a5890, s=0) at ../libs/temporal/temporal/timeline.h:55
#3 in ARDOUR::Automatable::non_realtime_locate(long) (this=0x55a12a980cc8, now=0) at ../libs/ardour/automatable.cc:421
#4 in ARDOUR::Route::non_realtime_locate(long) (this=0x55a12a980ae0, pos=0) at ../libs/ardour/route.cc:5462
#5 in ARDOUR::Session::non_realtime_stop(bool, int, bool&) (this=0x55a12e0cd000, abort=false, on_entry=1, finished=@0x7f94bc0a5e0f: true) at ../libs/ardour/session_transport.cc:1487
#6 in ARDOUR::Session::butler_transport_work(bool) (this=0x55a12e0cd000, have_process_lock=false) at ../libs/ardour/session_transport.cc:1153
#7 in ARDOUR::Butler::thread_work() (this=0x55a12f3b7000) at ../libs/ardour/butler.cc:222
#8 in ARDOUR::Butler::_thread_work(void*) (arg=0x55a12f3b7000) at ../libs/ardour/butler.cc:16
```
