avfilter/vf_curves: 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_curves.c

@@ -60,7 +60,7 @@ enum preset {
 
 typedef struct {
     const AVClass *class;
-    enum preset preset;
+    int preset;
     char *comp_points_str[NB_COMP + 1];
     char *comp_points_str_all;
     uint8_t graph[NB_COMP + 1][256];