Rework build and prepare for FW 21

[ViRb3]

This PR supersedes #10.

Changes and notes:

• Fully dockerized build with two parts:
  1. base image with static dependencies, which is cached and reused for faster builds
  2. SwitchWave source is mounted into the above and built there, allowing you to do incremental builds and debugging the environment
• gimp-bcn-conver.sh reworked:
  • GIMP 2 and 3 dual support with auto detection. Both verified and working via the dual Dockerfile setup.
  • No more swallowing of errors
  • Serialize with lock file due to race conditions which I hit twice in development
• GitHub Actions auto-build integration:
  • Artifacts are auto published for testing
  • You can find my test build here: https://github.com/ViRb3/SwitchWave/actions/runs/22315397785/artifacts/5620634166
• mpv submodule updated with a critical fix that causes playback crash with latest devkitpro: Fix crash using latest devkitpro mpv#1

I tested my build extensively, and can confirm the following works: video, audio, subtitles, network shares, local playback. If you want me to test anything else, please let me know!

This PR is in draft state since we need to merge the mpv PR first, then I can update to reference your submodule instead of my fork.

[ViRb3]

Switched to your mpv submodule. Ready for review.

[averne]

Great work, thanks a lot.
It's probably possible to work around this race condition in the gimp script by passing a template to mktemp. I'll see if I can make that work this evening.