Fix XM tremolo waveforms

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add OpenMPT TremoloWaveforms.xm test case

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Does vibrato in all frames imply in tremolo in all frames?

It seems consistent, but we should test that.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>