ffmpeg: default hwaccel_output_format to cuda when hwaccel is cuvid

This ensures old commandlines using -hwaccel cuvid don't break due to
the recent removal of the the cuvid-specific hwaccel bringup.

fftools/ffmpeg_opt.c

@@ -817,6 +817,25 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
             MATCH_PER_STREAM_OPT(top_field_first, i, ist->top_field_first, ic, st);
 
             MATCH_PER_STREAM_OPT(hwaccels, str, hwaccel, ic, st);
+            MATCH_PER_STREAM_OPT(hwaccel_output_formats, str,
+                                 hwaccel_output_format, ic, st);
+
+            if (!hwaccel_output_format && hwaccel && !strcmp(hwaccel, "cuvid")) {
+                av_log(NULL, AV_LOG_WARNING,
+                    "WARNING: defaulting hwaccel_output_format to cuda for compatibility "
+                    "with old commandlines. This behaviour is DEPRECATED and will be removed "
+                    "in the future. Please explicitly set \"-hwaccel_output_format cuda\".\n");
+                ist->hwaccel_output_format = AV_PIX_FMT_CUDA;
+            } else if (hwaccel_output_format) {
+                ist->hwaccel_output_format = av_get_pix_fmt(hwaccel_output_format);
+                if (ist->hwaccel_output_format == AV_PIX_FMT_NONE) {
+                    av_log(NULL, AV_LOG_FATAL, "Unrecognised hwaccel output "
+                           "format: %s", hwaccel_output_format);
+                }
+            } else {
+                ist->hwaccel_output_format = AV_PIX_FMT_NONE;
+            }
+
             if (hwaccel) {
                 // The NVDEC hwaccels use a CUDA device, so remap the name here.
                 if (!strcmp(hwaccel, "nvdec") || !strcmp(hwaccel, "cuvid"))
@@ -868,18 +887,6 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
                     exit_program(1);
             }
 
-            MATCH_PER_STREAM_OPT(hwaccel_output_formats, str,
-                                 hwaccel_output_format, ic, st);
-            if (hwaccel_output_format) {
-                ist->hwaccel_output_format = av_get_pix_fmt(hwaccel_output_format);
-                if (ist->hwaccel_output_format == AV_PIX_FMT_NONE) {
-                    av_log(NULL, AV_LOG_FATAL, "Unrecognised hwaccel output "
-                           "format: %s", hwaccel_output_format);
-                }
-            } else {
-                ist->hwaccel_output_format = AV_PIX_FMT_NONE;
-            }
-
             ist->hwaccel_pix_fmt = AV_PIX_FMT_NONE;
 
             break;