audio: Offer a hint for libsamplerate quality/speed tradeoff.

This defaults to the internal SDL resampler, since that's the likely default
without a system-wide install of libsamplerate, but those that need more can
tweak this.

src/audio/SDL_audio_c.h

@@ -39,6 +39,7 @@
 #ifdef HAVE_LIBSAMPLERATE_H
 #include "samplerate.h"
 extern SDL_bool SRC_available;
+extern int SRC_converter;
 extern SRC_STATE* (*SRC_src_new)(int converter_type, int channels, int *error);
 extern int (*SRC_src_process)(SRC_STATE *state, SRC_DATA *data);
 extern int (*SRC_src_reset)(SRC_STATE *state);