Improve more sas correctness

Primarily:
 * Significantly improves PCM handling, which now loops pretty much correctly, etc.
 * Matches a 32 sample lead in after keyon from tests.
 * Passes tests from hrydgard/pspautotests#138.
 * Adds some validation errors based on tests.

I *think* that VAG is also ending too early, probably ATRAC also, but I have not tested them yet so I left them where they are.  Assuming they work like PCM I plan to move the end handling.  It probably means the last few samples are clipped off currently.

-[Unknown]