swr: split out DSP functions.

DSP bits of swri_resample go into their own mini-DSP functions; DSP
init goes from a per-call branch in multiple_resample to a proper
DSP init routine; x86 bits go into x86/; swri_resample() moves out of
resample_template.c into resample.c because it's independent of DSP
code or sample type; multiple_resample() is simplified.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/Makefile

@@ -10,6 +10,7 @@ OBJS = audioconvert.o                        \
        dither.o                              \
        rematrix.o                            \
        resample.o                            \
+       resample_dsp.o                        \
        swresample.o                          \
 
 OBJS-$(CONFIG_LIBSOXR) += soxr_resample.o