Add a SDL 3 platform layer

[nmlgc]

It's going to be the future, cleans up the API in a few particularly annoying places, and adds a Vulkan backend to the 2D rendering API. I still went with SDL 2 for the Linux port because:

• SDL 3 wasn't stable when this port started in 2023, and still isn't as of August 2024.
• Consequently, it's not yet widely packaged by Linux distributions, and wide portability is the entire point of this port.
• I received funding to port specifically SDL 2 to Windows 98, which is obviously intended to help with other existing SDL 2 games and not just Shuusou Gyoku.

Due to the sweeping API changes between SDL 2 and 3, this will probably involve lots of code duplication…