Fix building with ffmpeg 5 and newer

[death-droid]

Newer version of FFMpeg have changed the returned value from the "avcodec_find_decoder_by_name" function.

This change should maintain support for older releases while also allowing it to compile with FFMpeg 5.0 and newer.

[Fredrum]

Hello and thanks for your interest to contribute.
I no longer have time to update and fix issues on this project as It's always work to test on several OS's and Hardware platforms and it all takes a bit too much time.

Currently this/my version of Chiaki, rpi02, is working off the RaspberryOS custom ffmpeg version. There was some talk when it was initially designed that it would be upstreamed to mainline ffmpeg. But later whaen I chatted to the guy making the RPiOS version he said that had met resistance and wasn't happening.

So the first thing to try would be if the official ffmpeg 5.0 now allows for the zero copy drm memory buffer to gl texture conversions that the custom rpi ffmpeg was doing. There might be some other specifics as well that the rpi version added.

Maybe your addition would be harmless even if the RPiOS ffmpeg was assumed but as you can see there's a bunch if things I'd want to look into and I just don't have the time unfortunately.