This allows reordering, duplication, and silencing of input channels.

Simplifies header dependencies by not including all other internal headers
in internal.h.

It will always be the number of samples in the input buffer, so just use that
directly instead of passing it as a separate parameter.

Signed-off-by: Mans Rullgard <mans@mansr.com>

Signed-off-by: Martin Storsjö <martin@martin.st>

This is a new library for audio sample format, channel layout, and sample rate
conversion.