avcodec/nvenc: only unregister input resources when absolutely needed

This reverts nvenc to old behaviour, which in some super rare edge cases
performs better.
The implication of this is that any potential API user who relies on
nvenc cleaning up every frames device resources after it's done using
them will have to change their usage pattern.

That should not be a problem, since pretty much every normal usage
pattern automatically implies that surfaces are reused from a common
pool, since constant re-allocation is also very expensive.

libavcodec/nvenc.c

@@ -1560,12 +1560,15 @@ static int nvenc_find_free_reg_resource(AVCodecContext *avctx)
     NV_ENCODE_API_FUNCTION_LIST *p_nvenc = &dl_fn->nvenc_funcs;
     NVENCSTATUS nv_status;
 
-    int i;
+    int i, first_round;
 
     if (ctx->nb_registered_frames == FF_ARRAY_ELEMS(ctx->registered_frames)) {
+        for (first_round = 1; first_round >= 0; first_round--) {
         for (i = 0; i < ctx->nb_registered_frames; i++) {
             if (!ctx->registered_frames[i].mapped) {
                 if (ctx->registered_frames[i].regptr) {
+                    if (first_round)
+                        continue;
                     nv_status = p_nvenc->nvEncUnregisterResource(ctx->nvencoder, ctx->registered_frames[i].regptr);
                     if (nv_status != NV_ENC_SUCCESS)
                         return nvenc_print_error(avctx, nv_status, "Failed unregistering unused input resource");
@@ -1575,6 +1578,7 @@ static int nvenc_find_free_reg_resource(AVCodecContext *avctx)
                 return i;
             }
         }
+        }
     } else {
         return ctx->nb_registered_frames++;
     }
@@ -1846,13 +1850,6 @@ static int process_output_surface(AVCodecContext *avctx, AVPacket *pkt, NvencSur
                 res = nvenc_print_error(avctx, nv_status, "Failed unmapping input resource");
                 goto error;
             }
-            nv_status = p_nvenc->nvEncUnregisterResource(ctx->nvencoder, ctx->registered_frames[tmpoutsurf->reg_idx].regptr);
-            if (nv_status != NV_ENC_SUCCESS) {
-                res = nvenc_print_error(avctx, nv_status, "Failed unregistering input resource");
-                goto error;
-            }
-            ctx->registered_frames[tmpoutsurf->reg_idx].ptr = NULL;
-            ctx->registered_frames[tmpoutsurf->reg_idx].regptr = NULL;
         } else if (ctx->registered_frames[tmpoutsurf->reg_idx].mapped < 0) {
             res = AVERROR_BUG;
             goto error;