avfilter/vf_fieldmatch: 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/vf_fieldmatch.c

@@ -86,13 +86,13 @@ typedef struct {
     /* options */
     int order;
     int ppsrc;
-    enum matching_mode mode;
+    int mode;                       ///< matching_mode
     int field;
     int mchroma;
     int y0, y1;
     int64_t scthresh;
     double scthresh_flt;
-    enum comb_matching_mode combmatch;
+    int combmatch;                  ///< comb_matching_mode
     int combdbg;
     int cthresh;
     int chroma;