Fix vibrato waveforms

Low value in square wave should be 0, not -255. Also fix ramp up waveform
index to 4, and leave 3 as the random waveform.

To check: is ramp up actually used by any format? And is vibrato depth
	ok in MOD/S3M/XM? It does seem correct in IT.

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