avfilter/vf_field: 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_field.c

@@ -33,7 +33,7 @@ enum FieldType { FIELD_TYPE_TOP = 0, FIELD_TYPE_BOTTOM };
 
 typedef struct {
     const AVClass *class;
-    enum FieldType type;
+    int type;                   ///< FieldType
     int nb_planes;              ///< number of planes of the current format
 } FieldContext;