macOS: always use Objective-C ARC (automatic ref counting).

Change Cocoa SDL_VideoData and SDL_WindowData implementations from C structs to Objective-C objects, since bridging between C and ObjC is easier that way.

CMakeLists.txt

@@ -2902,7 +2902,7 @@ if(ANDROID)
   target_include_directories(sdl-build-options INTERFACE "${ANDROID_NDK}/sources/android/cpufeatures")
 endif()
 
-if(IOS OR TVOS)
+if(APPLE)
   target_compile_options(sdl-build-options INTERFACE "-fobjc-arc")
 endif()