avfilter/vf_dnn_detect: replace magic number by enum value

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavfilter/vf_dnn_detect.c

@@ -43,12 +43,12 @@ typedef struct DnnDetectContext {
 #define OFFSET2(x) offsetof(DnnDetectContext, x)
 #define FLAGS AV_OPT_FLAG_FILTERING_PARAM | AV_OPT_FLAG_VIDEO_PARAM
 static const AVOption dnn_detect_options[] = {
-    { "dnn_backend", "DNN backend",                OFFSET(backend_type),     AV_OPT_TYPE_INT,       { .i64 = 2 },    INT_MIN, INT_MAX, FLAGS, "backend" },
+    { "dnn_backend", "DNN backend",                OFFSET(backend_type),     AV_OPT_TYPE_INT,       { .i64 = DNN_OV },    INT_MIN, INT_MAX, FLAGS, "backend" },
 #if (CONFIG_LIBTENSORFLOW == 1)
-    { "tensorflow",  "tensorflow backend flag",    0,                        AV_OPT_TYPE_CONST,     { .i64 = 1 },    0, 0, FLAGS, "backend" },
+    { "tensorflow",  "tensorflow backend flag",    0,                        AV_OPT_TYPE_CONST,     { .i64 = DNN_TF },    0, 0, FLAGS, "backend" },
 #endif
 #if (CONFIG_LIBOPENVINO == 1)
-    { "openvino",    "openvino backend flag",      0,                        AV_OPT_TYPE_CONST,     { .i64 = 2 },    0, 0, FLAGS, "backend" },
+    { "openvino",    "openvino backend flag",      0,                        AV_OPT_TYPE_CONST,     { .i64 = DNN_OV },    0, 0, FLAGS, "backend" },
 #endif
     DNN_COMMON_OPTIONS
     { "confidence",  "threshold of confidence",    OFFSET2(confidence),      AV_OPT_TYPE_FLOAT,     { .dbl = 0.5 },  0, 1, FLAGS},