resampler: Change interface, resampler may return the number of leftover frames

resampler: Change interface, resampler may return the number of leftover frames

some resampler implementations (e.g. libsamplerate and ffmpeg) do not consume
the entire input buffer; the impl_resample() function now has a return value
returning the number of frames in the input buffer not processed

these frames must be saved in appropriate buffer and presented together with
new input data

also change the parameter names from in_samples, out_samples to in_n_frames,
out_n_frames, respectively (n_frames = samples / channels)

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>