Fix AV1 demuxing

For AV1, the config packet must not be merged with the next non-config
packet.

This fixes the following error when passing --video-codec=av1:

> INFO: [FFmpeg] libdav1d 1.3.0
> ERROR: [FFmpeg] Unknown OBU type 0 of size 29393
> ERROR: [FFmpeg] Error parsing OBU data
> ERROR: Decoder 'video': could not send video packet: -1094995529

PR #4487 <https://github.com/Genymobile/scrcpy/pull/4487>