Relative mouse mode is tied to the window with keyboard focus

This isn't obvious, but makes sense when thinking about how games actually use it. This is also in line with how Windows mouse relative mode is implemented.

Fixes https://github.com/libsdl-org/SDL/issues/5340

src/video/cocoa/SDL_cocoamouse.m

@@ -262,7 +262,7 @@ Cocoa_SetRelativeMouseMode(SDL_bool enabled)
     /* We will re-apply the relative mode when the window gets focus, if it
      * doesn't have focus right now.
      */
-    SDL_Window *window = SDL_GetMouseFocus();
+    SDL_Window *window = SDL_GetKeyboardFocus();
     if (!window) {
         return 0;
     }