af_lavrresample: always normalize (libswresample is stupid)

libswresample doesn't normalize when remixing to a float format. This
will cause clipping due to float samples being out of the allowed range.
Fortunately this extremely bad default can be changed.

This does not happen with libavresample: it normalizes by default.

Fixes #1752.

audio/filter/af_lavrresample.c

@@ -200,6 +200,10 @@ static int configure_lavrr(struct af_instance *af, struct mp_audio *in,
 
     av_opt_set_double(s->avrctx, "cutoff",          s->ctx.cutoff, 0);
 
+#if HAVE_LIBSWRESAMPLE
+    av_opt_set_double(s->avrctx, "rematrix_maxval", 1.0, 0);
+#endif
+
     if (mp_set_avopts(af->log, s->avrctx, s->avopts) < 0)
         return AF_ERROR;