pcm_utils: pass only one buffer size to pcm_mix()

pcm_mix() might overflow the destination buffer if it is smaller than
the second buffer.  This is ok because the physical buffer size passed
by cross_fade_apply() is always big enough, but clutters pcm_mix()
with complicated length checks and contains a dangerous buffer
overflow pitfall.  Simplify pcm_mix()/pcm_add() and pass only the
smaller buffer size; let cross_fade_apply() do the memcpy().

src/pcm_utils.h

@@ -47,8 +47,8 @@ typedef struct _ConvState {
 void pcm_volumeChange(char *buffer, int bufferSize, const struct audio_format *format,
                       int volume);
 
-void pcm_mix(char *buffer1, const char *buffer2, size_t bufferSize1,
-             size_t bufferSize2, const struct audio_format *format, float portion1);
+void pcm_mix(char *buffer1, const char *buffer2, size_t size,
+             const struct audio_format *format, float portion1);
 
 size_t pcm_convertAudioFormat(const struct audio_format *inFormat,
 			      const char *inBuffer, size_t inSize,