Migrate backend from SDL 2 to SDL 3

[EliteMasterEric]

This pull request performs the following:

• Updates the project from SDL 2 to the latest version of SDL 3.
  • This lays the groundwork for later commits which provide new features based on these changes.
• Fixed a rendering issue caused by changes in how SDL 3.0 handles high-DPI displays.
• Window frame now displays in either black or white, depending on the user's system theme (dark/light).
  • Functionality to configure this, or retrieve the current system theme, are planned for later PRs.

[EliteMasterEric]

Testing with more sample projects, I have found the following issues:

• Stutters and major frame drops on an occasional basis, even when nothing particularly complex is happening.
• Issues with rendering of text in HaxeFlixel.

This will need to be resolved before development of SDL3 features like PreciseKeyEvent can continue.