Port ngage to header-only SDL_main + SDL_RunApp()

For some reason, ngage doesn't seem to be handled in any of the
build systems, so I couldn't add SDL_ngage_runapp.cpp to any buildscript

include/SDL3/SDL_main.h

@@ -110,6 +110,14 @@
 */
 #define SDL_MAIN_AVAILABLE
 
+#elif defined(__NGAGE__)
+
+/*
+   TODO: not sure if it should be SDL_MAIN_NEEDED, in SDL2 ngage had a
+        main implementation, but wasn't mentioned in SDL_main.h
+ */
+#define SDL_MAIN_AVAILABLE
+
 #endif
 #endif /* SDL_MAIN_HANDLED */
 
@@ -272,7 +280,7 @@ extern DECLSPEC void SDLCALL SDL_GDKSuspendComplete(void);
 #if !defined(SDL_MAIN_HANDLED) && !defined(SDL_MAIN_NOIMPL)
 /* include header-only SDL_main implementations */
 #if defined(__WIN32__) || defined(__GDK__) || defined(__IOS__) || defined(__TVOS__) \
-    || defined(__3DS__) /* TODO: other platforms */
+    || defined(__3DS__) || defined(__NGAGE__) /* TODO: other platforms */
 #include <SDL3/SDL_main_impl.h>
 #elif defined(__WINRT__) /* TODO: other C++ platforms */
 

include/SDL3/SDL_main_impl.h

@@ -156,7 +156,22 @@ int CALLBACK WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
 }
 
 /* end of WinRT impl */
-#elif defined(__IOS__) || defined(__TVOS__) || defined(__3DS__)
+#elif defined(__NGAGE__)
+
+/* same typedef as in ngage SDKs e32def.h */
+typedef signed int TInt;
+/* TODO: if it turns out that this only works when built as C++,
+         move __NGAGE__ into the C++ section in SDL_main.h */
+TInt E32Main()
+{
+    return SDL_RunApp(0, NULL, SDL_main, NULL);
+}
+
+/* end of __NGAGE__ impl */
+
+/* TODO: remaining platforms */
+
+#else /* platforms that use a standard main() and just call SDL_RunApp(), like iOS and 3DS */
 
 #include <SDL3/begin_code.h>
 
@@ -175,9 +190,7 @@ int main(int argc, char *argv[])
 
 #include <SDL3/close_code.h>
 
-/* end of __IOS__, __3DS__, __TVOS__ impls */
-
-/* TODO: remaining platforms */
+/* end of impls for standard-conforming platforms */
 
 #endif /* __WIN32__ etc */