Rewrite FFmpeg decoder selection logic to dynamically discover compatible decoders

This avoids having to maintain a hardcoded list of all known H.264/HEVC/AV1 decoders
and ensures new/unknown decoders will just work automatically (assuming we have
a renderer that can process their output pixel format).

app/streaming/video/ffmpeg.h

@@ -40,8 +40,9 @@ private:
 
     bool createFrontendRenderer(PDECODER_PARAMETERS params, bool useAlternateFrontend);
 
-    bool tryInitializeRendererForDecoderByName(const char* decoderName,
-                                               PDECODER_PARAMETERS params);
+    bool tryInitializeRendererForUnknownDecoder(const AVCodec* decoder,
+                                                PDECODER_PARAMETERS params,
+                                                bool tryHwAccel);
 
     bool tryInitializeRenderer(const AVCodec* decoder,
                                PDECODER_PARAMETERS params,