ffmpeg: Use ch_layout in favor of removed channel_layout and channels

[darktohka]

Issue description

In FFmpeg/FFmpeg@65ddc74, the channel_layout and channels members in AVCodecParameters and AVCodecContext were removed. This corresponds to the libavformat version 60.25.100.

They have been deprecated since libavformat version 59.18.101: FFmpeg/FFmpeg@548aeb9

The avcodec_close function has also been deprecated for a while, it is unnecessary to call both avcodec_close and avcodec_free_context.

Solution description

When compiling with a version older than 59.18.101, use the old channel_layout and channels members. For versions newer than 59.18.101, use the new ch_layout member.

Checklist

I have done my best to ensure that…

• …I have familiarized myself with the CONTRIBUTING.md file
• …this change follows the coding style and design patterns of the codebase
• …I own the intellectual property rights to this code
• …the intent of this change is clearly explained
• …existing uses of the Panda3D API are not broken
• …the changed code is adequately covered by the test suite, where possible.

[darktohka]

The original version of this PR had a bug: it was not using the new chlayout interface for swresample. Now the API usage is fully corrected.