ffmpeg: Introduce 'cuda' hwaccel and deprecate 'cuvid'

We want to switch to using the generic hwaccel codepath but doing that
introduces a requirement to pass '-hwaccel_output_format cuda' on the
command line to turn on full transcoding.

That breaks compatibility, so we'd rather deprecate than cut over
directly.

But this change doesn't seem to be working. I can specify one of
the two hwaccels but the 'cuda' one doesn't seem to work. The context
does not get shared between the decoder and encoder. If I reverse
the declaration order of the two hwaccels, then 'cuda' works and
'cuvid' does not (cuvid_init isn't even called). I assume this is
related to having both of them say they handle the same pix_fmt.

ffmpeg.h

@@ -67,6 +67,7 @@ enum HWAccelID {
     HWACCEL_VIDEOTOOLBOX,
     HWACCEL_QSV,
     HWACCEL_VAAPI,
+    HWACCEL_CUDA,
     HWACCEL_CUVID,
 };
 

ffmpeg_cuvid.c

@@ -33,6 +33,12 @@ int cuvid_init(AVCodecContext *avctx)
     AVHWFramesContext *frames_ctx;
     int ret;
 
+    av_log(avctx, AV_LOG_WARNING,
+           "The 'cuvid' hwaccel is deprecated in favour of the 'cuda' "
+           "hwaccel. When switching to 'cuda', you must specify the "
+           "output format using '-hwaccel_output_format cuda' to get "
+           "the same transcoding configuration as 'cuvid'.\n");
+
     av_log(avctx, AV_LOG_VERBOSE, "Initializing cuvid hwaccel\n");
 
     if (!hw_device_ctx) {

ffmpeg_opt.c

@@ -93,6 +93,8 @@ const HWAccel hwaccels[] = {
 #if CONFIG_CUVID
     { "cuvid", cuvid_init, HWACCEL_CUVID, AV_PIX_FMT_CUDA,
       AV_HWDEVICE_TYPE_NONE },
+    { "cuda", hwaccel_decode_init, HWACCEL_CUDA, AV_PIX_FMT_CUDA,
+      AV_HWDEVICE_TYPE_CUDA },
 #endif
     { 0 },
 };