audio: enable pitch correction by default when playing fast

Apparently this is what users want. When playing with normal speed,
nothing is done. When playing slower than normal, resampling is used
instead, because scaletempo (which does the pitch correction) adds
too many artifacts.

options/options.c

@@ -249,6 +249,8 @@ const m_option_t mp_opts[] = {
     OPT_DOUBLE("speed", playback_speed, M_OPT_RANGE | M_OPT_FIXED,
                .min = 0.01, .max = 100.0),
 
+    OPT_FLAG("audio-pitch-correction", pitch_correction, 0),
+
     // set a-v distance
     OPT_FLOATRANGE("audio-delay", audio_delay, 0, -100.0, 100.0),
 
@@ -639,6 +641,7 @@ const struct MPOpts mp_default_opts = {
     .audio_output_channels = MP_CHMAP_INIT_STEREO,
     .audio_output_format = 0,  // AF_FORMAT_UNKNOWN
     .playback_speed = 1.,
+    .pitch_correction = 1,
     .movie_aspect = -1.,
     .field_dominance = -1,
     .sub_auto = 0,

options/options.h

@@ -205,6 +205,7 @@ typedef struct MPOpts {
     int force_srate;
     int dtshd;
     double playback_speed;
+    int pitch_correction;
     struct m_obj_settings *vf_settings, *vf_defs;
     struct m_obj_settings *af_settings, *af_defs;
     int deinterlace;