Use Flathub submodules for SDL2 and Libdecor

[orowith2os]

Makes the build process quite a bit cleaner.

Also, does it make sense for Moonlight to still build ffmpeg manually? can it not use the ffmpeg extension(s) provided by Flathub?

CC (?) @cgutman

[flathubbot]

Started test build 10637

[flathubbot]

Build 10637 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/123089/com.moonlight_stream.Moonlight.flatpakref

[flathubbot]

Started test build 10769

[flathubbot]

Build 10769 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/123223/com.moonlight_stream.Moonlight.flatpakref

[cgutman]

@orowith2os Is the intention to keep bumping that shared-module to match the latest SDL2 series release? Are you sticking with 2.26.x patches or will you bump to 2.28.x when it releases?

Being such a complex SDL2-based application, Moonlight tends to run into issues that require fixes from bleeding-edge SDL2 commits (things like new APIs, controller compatibility fixes, Wayland fixes, window management fixes, etc). I think 2.26.x is fine for us now, but it's possible we may have to go back to building ourselves if we run into issues like that again.

Also, does it make sense for Moonlight to still build ffmpeg manually? can it not use the ffmpeg extension(s) provided by Flathub?

Yes, I think it still makes sense to build it ourselves. We have non-upstreamed patches that we carry in our fork and the FFmpeg-full extension doesn't have AV1 or NVDEC enabled.

[orowith2os]

The main modules were already bumped to the latest SDL2 version, iirc, so I'll update this PR for that. Considering SDL3 is currently being developed, I don't think SDL2 will be getting much action anyways.

[cgutman]

The main modules were already bumped to the latest SDL2 version, iirc, so I'll update this PR for that.

OK, thanks. I'll merge once it's updated.

Considering SDL3 is currently being developed, I don't think SDL2 will be getting much action anyways.

It's still pretty active for bugfixes: https://github.com/libsdl-org/SDL/commits/SDL2