Pass some state to convert and channelmix functions. This makes it
possible to select per channel optimized convert functions but
also makes it possible to implement noise shaping later.
Pass the channelmix matrix and volume in the state.
Handle specialized 2 channel s16 -> f32 conversion
Improve the allocators to always align the buffer memory to the
requested alignment
Use aligned read and writes for sse functions and check alignment,
optionally falling back to unaligned path.
Add more tests and benchmark cases
Check and warn for misaligned memory in plugins.
