lavc/libmp3lame: add support for cutoff

Pass the cutoff option from lavc's avcodec_options[] to libmp3lame's
lowpass option, without allowing to adjust its default behavior.
Signed-off-by: Moritz Barsnick <barsnick@gmx.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/encoders.texi

@@ -817,6 +817,10 @@ Set algorithm quality. Valid arguments are integers in the 0-9 range,
 with 0 meaning highest quality but slowest, and 9 meaning fastest
 while producing the worst quality.
 
+@item cutoff (@emph{--lowpass})
+Set lowpass cutoff frequency. If unspecified, the encoder dynamically
+adjusts the cutoff.
+
 @item reservoir
 Enable use of bit reservoir when set to 1. Default value is 1. LAME
 has this enabled by default, but can be overridden by use

libavcodec/libmp3lame.c

@@ -125,6 +125,10 @@ static av_cold int mp3lame_encode_init(AVCodecContext *avctx)
         }
     }
 
+    /* lowpass cutoff frequency */
+    if (avctx->cutoff)
+        lame_set_lowpassfreq(s->gfp, avctx->cutoff);
+
     /* do not get a Xing VBR header frame from LAME */
     lame_set_bWriteVbrTag(s->gfp,0);