Fix to silent & generic errors when loading videos with unsupported codecs

[shoyguer]

Currently when av_find_best_stream returns AVERROR_DECODER_NOT_FOUND, the container is parsed and the stream exists. The codec is just not registered in this FFmpeg build.

The old code treated this the same as a missing stream. It fell through to "Couldn't find video stream: Decoder not found", with no hint about which codec is the problem. The playback object returned null and VideoStreamPlayer failed silently.

Changes

• Adds a dedicated AVERROR_DECODER_NOT_FOUND handler in prepare_decoding(). It re-probes ignoring decoder availability, names the codec, and tries a named fallback before failing.
• Renames _codec_id_to_libvpx to _codec_id_to_preferred_decoder_name since the function now covers more than VP8/VP9.
• Extends the fallback table with AV_CODEC_ID_HEVC ("hevc") and AV_CODEC_ID_AV1 ("libaom-av1"). If a future build includes those decoders, the plugin picks them up with no code change.
• The error now names the codec instead of giving a generic message.

Before

These is how errors were shown, for reference:

Couldn't find video stream: Decoder not found
Plugin returned null playback

After (unsupported codec, no fallback available)

Example to how errors are shown now:

No decoder found for 'hevc': codec not supported by this FFmpeg build.

After (supported named decoder found)

No default decoder for 'vp9', retrying with 'libvpx-vp9'.

With these changes, now my .mp4 videos that were NOT being played.
I used this export properties on Davinci Resolve

Info about the videos after Davinci:
video.txt

Info about the videos after I used Handbrake to compress them further:
videohb.txt
(all information taken from MediaInfo).

I was getting these errors before:

E 0:00:07:010   scene_loader.gd:184 @ _fade_in(): Couldn't find video stream: Decoder not found
  <C++ Error>   Condition "stream_index < 0" is true.
  <C++ Source>  D:\a\EIRTeam.FFmpeg\EIRTeam.FFmpeg\video_decoder.cpp:127 @ VideoDecoder::prepare_decoding()
  <Stack Trace> scene_loader.gd:184 @ _fade_in()
                scene_loader.gd:319 @ load_scene()
                intro.gd:45 @ <anonymous lambda>()


E 0:00:07:010   scene_loader.gd:184 @ _fade_in(): Plugin returned null playback
  <C++ Error>   Condition "ret.is_null()" is true. Returning: nullptr
  <C++ Source>  scene/resources/video_stream.cpp:161 @ instantiate_playback()
  <Stack Trace> scene_loader.gd:184 @ _fade_in()
                scene_loader.gd:319 @ load_scene()
                intro.gd:45 @ <anonymous lambda>()

Now everything runs perfectly, with the same videos.

Hope it helps, as it helped me. If you find ANY issues, please let me know, so I can fix them ASAP.
Thanks for your time and for this amazing plugin!
😄