resampler: Add support for resamplers that consume less data than asked.

resampler: Add support for resamplers that consume less data than asked.

libsamplerate_resample() assumed that src_process() would
always consume the whole input buffer. That was an invalid
assumption leading to crashes.

This patch adds a leftover memchunk for storing any
non-consumed input. When pa_resampler_run() is called next
time, the leftover is prepended to the new input.

Changes in v3:
 - Make the calculations in pa_resampler_result() and
   pa_resampler_max_block_size() more readable and more
   correct.
 - Rework the leftover storing: instead of using a dedicated
   buffer for it, store it in the beginning of remap_buf.
   This can avoid some memory copying. (The idea was
   suggested by Wang Xingchao.)
 - Use a generic save_leftover() function instead of doing
   the leftover copying in the resampler implementation.
 - Use the leftover logic also with the speex and ffmpeg
   resamplers.
   [ed: dropped the speex bit since the API guarantees that
    it will consume everything -- Arun]

Changes in v2:
 - If add_leftover() is called with zero-length input while
   the leftover length is non-zero, we don't try to acquire
   the input memblock.
 - Instead of taking a reference to the original input in
   libsamplerate_resample(), we copy the leftover data to a
   new memblock. This is done, because otherwise, if the
   input is one of the internal buffers, the data can get
   overwritten before reading it in add_leftover().
 - Store add_leftover_buf size in bytes instead of samples
   (more convenient, but less consistent with other code).

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=47156