Use stdbool for SDL_bool

This helps the compiler warn people when they're doing something like "if (SDL_Init(0) < 0)"

cmake/test/main.swift

@@ -2,7 +2,7 @@
 
 import SDL3
 
-guard SDL_Init(SDL_INIT_VIDEO) != 0 else {
+guard SDL_Init(SDL_INIT_VIDEO) else {
   fatalError("SDL_Init error: \(String(cString: SDL_GetError()))")
 }