Improve pitch handling in sas

Based on testing:
 * Fixes the 32-sample lead-in on keyon with various pitch values.
 * Completes the vag sample before ending the envelope.  This is likely to (slightly) affect a lot of games, but is correct per my tests - it should process the final samples at the normal height before ending the envelope.
 * Resets sampleFrac at key on - otherwise, a new keyon could persist the old pitch fraction.
 * Does basic interpolation of the samples.  I'm not sure if this is exactly right (especially at irregular pitches), but it seems better.

-[Unknown]