Files that have many tracks, each with tempo information
were near impossible to load (30+ mins on modern 4.2Ghz CPU!),
because tempo is parsed incrementally:
```
For each new track:
for each new tempo-event:
rewind()
for each loaded track so far:
for each event on this track so far
```
This reduces the complexity from O(tracks^2 * tempos^2)
to O(tracks * tempos).
"Come Thou Fount Tempo Map.mid" has 238 Tracks and 56168 total
Tempo Changes (236 per track). This now requires only 56168 iterations
in smf_create_tempo_map_and_compute_seconds, rather than 1.64e+9
iterations
This significantly speeds up parsing MIDI files with complex
tempo-maps. e.g. "Black MIDI Trilogy_2.mid" has 24134 Tempo
changes. Prior to this commit parsing that file took over 5 minutes.
now it loads in under one seconds (libsmf only; libardour still
add overhead, and now needs about 30-40 seconds, previously
it took about 10 mins).
The problem was that every call to `smf_track_add_event_pulses()`
calls `seconds_from_pulses()` which calls `smf_get_tempo_by_seconds()`
which iterates over the tempo-map:
for every midi-event { for ever tempo until that midi-event {..} }
This does not scale to 3.5M events and 24k tempo-changes.
