use in-tree SDL

[tophyr]

Pull Request Type

• [ ] GitHub Workflow changes
• [ ] Documentation or Wiki changes
• [x] Build and Dependency changes
• [ ] Runtime changes
  • [ ] Render changes
  • [ ] Audio changes
  • [ ] Input changes
  • [ ] Network changes
  • [ ] Other changes

Description

Use SDL as a git submodule, and incorporate the project directly into our build instead of relying on a prebuilt library provided by a package manager or system configuration. This will enable us to build SDL precisely to our needs, for our build target.

Checklist

• [x] I have tested my changes locally and verified that they work as intended.
• [x] I have documented any new or modified functionality.
• [x] I have reviewed the changes to ensure they do not introduce any unnecessary complexity or duplicate code.
• [x] I understand that by submitting this pull request, I am agreeing to license my contributions under the project's license.

[tophyr]

Will need reconciliation with #457

[Lgt2x]

why not using VCPKG? And we probably don't want SDL unstable master branch

[Lgt2x]

As discussed on Discord, I'd prefer a unified VCPKG-based solution for our external packages. We don't have a tangible reason to require a personalized (and master branch) SDL2 build right now. VCPKG can apparently be used for Android, and compile libraries in debug mode if we need to.