Fix IT envelopes with no envelope points

When loading an IT module, if the envelope is on but no envelope points
are defined, define one envelope point. See BeautifulOnes.it.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix anticlick when setting sample offset

Smooth anticlick when setting sample position to compensate rampdowns.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Comment out IRQ code in YM3812 emulator

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix MED4 pattern reading

Fix line and effects mask for patterns with more than 64 lines. See
MED.(Ready Robot)Synth-a-sysmic.med (reported by Simon Spiers).

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix MED portamento and vibrato

Despite the fact that the MED docs say that effect 3 is vibrato, it's
in fact portamento in MED4 like in Soundtracker. Also change MED3 and MED2.

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