Revamped soft-filter API

Multithreaded and SIMD-aware API.
Here's a sample filter plugin: https://gist.github.com/Themaister/10696355

Multithreading is done via worker thread packets, no threading API stuff in the filters themselves.

NOTE: I'm not really a fan of CPU-side filtering, but at least the old bSNES cruft is gone now ...