avfilter/avf_avectorscope: Change enums to int, which are accessed via AVOption as int

This fixes depending on implementation defined behavior
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/avf_avectorscope.c

@@ -44,7 +44,7 @@ typedef struct AudioVectorScopeContext {
     AVFrame *outpicref;
     int w, h;
     int hw, hh;
-    enum VectorScopeMode mode;
+    int mode;
     int contrast[3];
     int fade[3];
     double zoom;