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
cd53301d06