mix: Fix mixing of S24 samples stored as S32RE

pa_mix_s24_32re_c() should advance m->ptr by sizeof(int32_t)

http://lists.freedesktop.org/archives/pulseaudio-discuss/2014-July/020998.html

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
sconv: Use optimized conversion function for both directions

for example, the conversion function for
convert_from_float32ne(PA_SAMPLE_S16LE) can also be used for
convert_to_s16ne(PA_SAMPLE_FLOAT32LE)

v2: ARM can potentially be big- or little endian; only apply
optimization on LE based on WORDS_BIGENDIAN #define (thanks, Tanu)

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
tests: Cleanup mix-test

indentation and use of fabsf() for floats

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
endianmacros: Fix PA_MAYBE_INT16_SWAP() macro

PA_MAYBE_INT16_SWAP() should call PA_INT16_SWAP(), not PA_INT32_SWAP

PA_MAYBE_INT16_SWAP() is not used (yet), so no big deal :)

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