libavfilter/dnn: Add scale and mean preprocess to openvino backend

Dnn models has different data preprocess requirements. Scale and mean
parameters are added to preprocess input data.

Signed-off-by: Wenbin Chen <wenbin.chen@intel.com>

libavfilter/dnn_interface.h

@@ -69,6 +69,8 @@ typedef struct DNNData{
     DNNDataType dt;
     DNNColorOrder order;
     DNNLayout layout;
+    float scale;
+    float mean;
 } DNNData;
 
 typedef struct DNNExecBaseParams {