frei0r: handle string params

This is needed for some of the frei0r filters (facebl0r and facedetect) which accept string parameters.

Fixes the issue described here:
http://www.ffmpeg-archive.org/Unable-to-set-ffmpeg-frei0r-facedetect-filter-parameter-td4680190.htmlSigned-off-by: Raphael Graf <r@undefined.ch>

libavfilter/vf_frei0r.c

@@ -93,6 +93,7 @@ static int set_param(AVFilterContext *ctx, f0r_param_info_t info, int index, cha
         double d;
         f0r_param_color_t col;
         f0r_param_position_t pos;
+        f0r_param_string *str;
     } val;
     char *tail;
     uint8_t rgba[4];
@@ -126,6 +127,11 @@ static int set_param(AVFilterContext *ctx, f0r_param_info_t info, int index, cha
         break;
     }
 
+    case F0R_PARAM_STRING:
+        val.str = param;
+        break;
+    }
+
     s->set_param_value(s->instance, &val, index);
     return 0;