WinRT: made SDL_winrt_main.cpp not have to be compiled as C++/CX (via the /ZW compiler flag)

This file can still be compiled as C++/CX, however that is now optional/not-required.

src/main/winrt/SDL_winrt_main.cpp

@@ -1,14 +1,36 @@
 
-//#include "pch.h"
+#include <wrl.h>
 
-// The app's C-style main will be passed into SDL.dll as a function
-// pointer, and called at the appropriate time.
+/* The app's C-style main will be passed into SDL.dll as a function
+   pointer, and called at the appropriate time.
+*/
 typedef int (*SDLmain_MainFunction)(int, char **);
 extern __declspec(dllimport) int SDL_WinRT_RunApplication(SDLmain_MainFunction mainFunction);
 extern "C" int SDL_main(int, char **);
 
-[Platform::MTAThread]
-int main(Platform::Array<Platform::String^>^)
+/* Prevent MSVC++ from warning about threading models when defining our
+   custom WinMain.  The threading model will instead be set via a direct
+   call to Windows::Foundation::Initialize (rather than via an attributed
+   function).
+
+   To note, this warning (C4447) does not seem to come up unless this file
+   is compiled with C++/CX enabled (via the /ZW compiler flag).
+*/
+#ifdef _MSC_VER
+#pragma warning(disable:4447)
+#endif
+
+/* Make sure the function to initialize the Windows Runtime gets linked in. */
+#ifdef _MSC_VER
+#pragma comment(lib, "runtimeobject.lib")
+#endif
+
+int CALLBACK WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
 {
-    return SDL_WinRT_RunApplication(SDL_main);
+    if (FAILED(Windows::Foundation::Initialize(RO_INIT_MULTITHREADED))) {
+        return 1;
+    }
+
+    SDL_WinRT_RunApplication(SDL_main);
+    return 0;
 }