swresample: Use channel count in rematrix initialization

Rematrixing supports up to 64 channels. However, there is only a limited number of channel layouts defined. Since the in/out channel count is currently obtained from the channel layout, for undefined layouts (e.g. for 9, 10, 11 channels etc.) the rematrixing fails.

This patch changes rematrix init methods to use in (used) and out channel count directly instead of computing it from channel layout.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>