Refactor sample offset value and memory data fields

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix set sample offset effect

Recently i confirmed with Protracker 2.3B that indeed a non-looped sample
should mute if the offset is higher than len, but what's more interesting is
how looped samples should behave...

First i thought that your original code (set to 0) is correct, until i
recently heard "ftp://ftp.modland.com/pub/modules/Substance/landing on planet
earth.mod" which seem to prove that the offset of looped-samples should be
set to the LOOP START instead !?. Just try to play this mod, it sounds so
funny without the patch :)

Signed-off-by: Dennis Lindroos <denafcm@gmail.com>
Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Update changelog

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