more work toward fixing the static - still not fully fixed

This PR includes the following changes

* filtering is now done on the PRE-MIX source buffer, so that we don't get the modulation and multiple damping effect with multiple sources

* I've moved the filter object to a single filter per mixer, and added a call to reset() before usage... this reduces the likelihood of transient and latent data in the filter from multiple disparate sources and calls.

The problem of "loud static" when rotated off from the source still exists.