Add CPU path for transparent encoding and decoding, add transparency support to FFmpeg-backed VP9 decoder and encoder

packages/server/src/misc.ts

@@ -229,3 +229,6 @@ export const getChannelLayout = (numChannels: number): NodeAv.ChannelLayout => {
 		default: return { nbChannels: numChannels, order: NodeAv.AV_CHANNEL_ORDER_UNSPEC, mask: 0n };
 	}
 };
+
+// The value is incorrect in the node-av source code, so we do:
+export const LIBVPX_VP9 = 'libvpx-vp9' as (NodeAv.FFEncoderCodec & NodeAv.FFDecoderCodec);