avcodec/nvenc: provide nvenc with monotonic frame index

libavcodec/nvenc.c

@@ -2784,6 +2784,7 @@ static int nvenc_send_frame(AVCodecContext *avctx, const AVFrame *frame)
             pic_params.encodePicFlags = 0;
         }
 
+        pic_params.frameIdx = ctx->frame_idx_counter++;
         pic_params.inputTimeStamp = frame->pts;
 
         if (ctx->extra_sei) {

libavcodec/nvenc.h

@@ -226,6 +226,8 @@ typedef struct NvencContext
 
     void *nvencoder;
 
+    uint32_t frame_idx_counter;
+
     int preset;
     int profile;
     int level;