Apply envelope & panning after biquad filtering.
This may fix the weird popping problem. It also reduces the number
of operations that are done "in stereo", so it could help performance.
It also fixes a previously unnoticed problem where a ring-modulated
waveform had 2x the amplitude of an un-modulated waveform.
The test differences look large but it's because some values got changed
in the LSB after the mathematical divisions were moved around.
