lavr: Add "resample_cutoff" option as a duplicate of "cutoff"

Avoids an option name conflict with libavcodec, which is needed in order
to work properly with avconv.

libavresample/options.c

@@ -54,6 +54,8 @@ static const AVOption options[] = {
     { "phase_shift",            "Resampling Phase Shift",   OFFSET(phase_shift),            AV_OPT_TYPE_INT,    { .i64 = 10             }, 0,                    30, /* ??? */           PARAM },
     { "linear_interp",          "Use Linear Interpolation", OFFSET(linear_interp),          AV_OPT_TYPE_INT,    { .i64 = 0              }, 0,                    1,                      PARAM },
     { "cutoff",                 "Cutoff Frequency Ratio",   OFFSET(cutoff),                 AV_OPT_TYPE_DOUBLE, { .dbl = 0.8            }, 0.0,                  1.0,                    PARAM },
+    /* duplicate option in order to work with avconv */
+    { "resample_cutoff",        "Cutoff Frequency Ratio",   OFFSET(cutoff),                 AV_OPT_TYPE_DOUBLE, { .dbl = 0.8            }, 0.0,                  1.0,                    PARAM },
     { "matrix_encoding",        "Matrixed Stereo Encoding", OFFSET(matrix_encoding),        AV_OPT_TYPE_INT,    {.i64 =  AV_MATRIX_ENCODING_NONE}, AV_MATRIX_ENCODING_NONE,     AV_MATRIX_ENCODING_NB-1, PARAM, "matrix_encoding" },
         { "none",  "None",               0, AV_OPT_TYPE_CONST, { .i64 = AV_MATRIX_ENCODING_NONE  }, INT_MIN, INT_MAX, PARAM, "matrix_encoding" },
         { "dolby", "Dolby",              0, AV_OPT_TYPE_CONST, { .i64 = AV_MATRIX_ENCODING_DOLBY }, INT_MIN, INT_MAX, PARAM, "matrix_encoding" },