More robust tempo handling in midi files.

Added a check to ignore multiple identical tempo changes in a midi file.

Theoretically, more than one tempo change can not occur at the exact same time. However, if there are multiple tempo changes at the exact same time _but with the same target tempo_ there is not really any reason to bail out.

I had some songs that were being ignored because of two identical tempo changes near the end of the song, these songs play just fine with this patch.