feat: use cmake's builtin FindSDL

[xiaoxiangmoe]

FindSDL2.cmake is added in cmake 3.19.

Current version ./cmake/FindSDL2.cmake has some bug that can't use it in nixos. We should use cmake builtin version FindSDL instead.

[xiaoxiangmoe]

@OmniBlade Can you help me to fix this windows ci bug? I don't have windows pc now.

[OmniBlade]

Unfortunately that is only a find module for SDL, not SDL2 which has never had an official find module in CMake. The reason it works on Linux is because SDL2 now ships with appropriate files of its own which on Linux get installed in a standard location CMake knows to look in. This isn't true on Windows hence the failure on the CI there.

The options are either 1. Write/update the find module to better work with Nix or 2. Point CMake to the correct location on a Windows install (and potentially also macOS) to look for the CMake info find_package searches for.