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
see also https://github.com/stump/libsmf/pull/7
* Fix validity checks of escaped data
* Handle non-EOT-terminated tracks.
* Fix buffer overflow on tempo change event
* Fix memory leaks in case loading fails
* Fix a logic errors in extract_escaped_event()
* Fix the assertion problem `is_sysex_byte(status)`
* Make libsmf more tolerant to malformed MIDI files.
(fixes import of files generated by NoteEdit)
