af_volume: implement replaygain pre-amplification

This adds a new "replaygain_preamp" option to the filter, and simply adds its
value to the replaygain gain value.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

doc/filters.texi

@@ -633,6 +633,12 @@ Prefer track gain, if present.
 @item album
 Prefer album gain, if present.
 @end table
+
+@item replaygain_preamp
+Pre-amplification gain in dB to apply to the selected replaygain gain.
+
+Default value for @var{replaygain_preamp} is 0.0.
+
 @end table
 
 @subsection Examples

libavfilter/af_volume.c

@@ -59,6 +59,8 @@ static const AVOption options[] = {
         { "ignore", "replaygain side data is ignored", 0, AV_OPT_TYPE_CONST, { .i64 = REPLAYGAIN_IGNORE }, 0, 0, A, "replaygain" },
         { "track",  "track gain is preferred",         0, AV_OPT_TYPE_CONST, { .i64 = REPLAYGAIN_TRACK  }, 0, 0, A, "replaygain" },
         { "album",  "album gain is preferred",         0, AV_OPT_TYPE_CONST, { .i64 = REPLAYGAIN_ALBUM  }, 0, 0, A, "replaygain" },
+    { "replaygain_preamp", "Apply replaygain pre-amplification",
+            OFFSET(replaygain_preamp), AV_OPT_TYPE_DOUBLE, { .dbl = 0.0 }, -15.0, 15.0, A },
     { NULL },
 };
 
@@ -262,7 +264,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *buf)
             av_log(inlink->dst, AV_LOG_VERBOSE,
                    "Using gain %f dB from replaygain side data.\n", g);
 
-            vol->volume   = pow(10, g / 20);
+            vol->volume   = pow(10, (g + vol->replaygain_preamp) / 20);
             vol->volume_i = (int)(vol->volume * 256 + 0.5);
 
             volume_init(vol);

libavfilter/af_volume.h

@@ -47,6 +47,7 @@ typedef struct VolumeContext {
     AVFloatDSPContext fdsp;
     enum PrecisionType precision;
     enum ReplayGainType replaygain;
+    double replaygain_preamp;
     double volume;
     int    volume_i;
     int    channels;