SDL3 port

[cxong]

Migration guide: https://github.com/libsdl-org/SDL/blob/main/docs/README-migration.md

Some features to take advantage of:

• Main callback (probably a good idea for long-term flexibility, since many frontends like emscripten want to use callbacks)
• Audio pitch: replace manually generated pitch variations
• Default audio devices i.e. hot plugging audio devices
• Clipboard API (copy a rendering of the entire game map as an image; very silly idea but could be fun)

Features that need more investigation to see what can benefit C-Dogs SDL:

• GPU/shader support
• File dialogs (not needed for now; we used to use a library for native dialogs but went back to a built-in file dialog due to compat issues)
• Filesystem API (we use tinydir, so more info required - is there anything to gain here?)
• Storage API (for storing save/config files? Would it be better? Needs to be backwards-compatible)
• Properties API (we already have Config which uses key/values, probably not gaining much but would be good to note the differences)
• Better keyboard input (???)
• App metadata API (???)