avfilter/vf_lut3d: 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_lut3d.c

@@ -58,7 +58,7 @@ struct rgbvec {
 
 typedef struct LUT3DContext {
     const AVClass *class;
-    enum interp_mode interpolation;
+    int interpolation;          ///<interp_mode
     char *file;
     uint8_t rgba_map[4];
     int step;