Reset dither state when API path changes.

The caller can switch between the fixed/float APIs and the
 stereo/multichannel APIs on every call.
Detect this and reset the dither state to avoid potentially
 injecting noise from a very out-of-date state when switching from
 fixed to float back to fixed, or from the wrong channels when
 switching between stereo and multichannel.
Normal applications won't do this stuff, but we should be
 well-behaved if they do.