FFMPEG 7.0 fixes

(I've verified that gbp buildpackage succeeds with these.)

Backport https://github.com/mpv-player/mpv/pull/13665

These changes will be needed for ffmpeg 7.0, otherwise there's no sound:

AO: [pipewire] 48000Hz 5.1(side) 6ch floatp
VO: [gpu] 1920x960 yuv420p10
[ffmpeg] SWR: Input channel layout "" is invalid or unsupported.
[swresample] Cannot open Libavresample context.
[swresample] libswresample failed to initialize.
Cannot convert decoder/filter output to any format supported by the
output.
Audio: no audio

Refs. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1068660