avfilter/vf_setfield: Change enum to int, which is accessed via AVOption as int

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

libavfilter/vf_setfield.c

@@ -37,7 +37,7 @@ enum SetFieldMode {
 
 typedef struct {
     const AVClass *class;
-    enum SetFieldMode mode;
+    int mode;                   ///< SetFieldMode
 } SetFieldContext;
 
 #define OFFSET(x) offsetof(SetFieldContext, x)