avfilter/vf_stereo3d: 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_stereo3d.c

@@ -63,7 +63,7 @@ enum StereoCode {
 };
 
 typedef struct StereoComponent {
-    enum StereoCode format;
+    int format;                 ///< StereoCode
     int width, height;
     int off_left, off_right;
     int off_lstep, off_rstep;