PortNintendo 3DS SDL_main to header-only + SDL_N3DSRunApp()

and move the #undef main and #define main SDL_main to the start/end of
SDL_main_impl.h instead of repeating it in every platform implementation

Thanks to SDL_N3DSRunApp we don't need the #include <3ds.h> in
SDL_main_impl.h - that caused conflicts with testthread.c, because both
have (different) ThreadFunc typedefs.

src/main/n3ds/SDL_n3ds_main.c

@@ -17,44 +17,9 @@
   2. Altered source versions must be plainly marked as such, and must not be
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
+
+   Nothing to do here, the code moved into SDL_main_impl.h
+    TODO: remove this file
 */
-#include <SDL3/SDL.h>
-#include <SDL3/SDL_main.h> /* until this SDL_main impl is converted to header-only.. */
-
-#ifdef __3DS__
-
-#include <3ds.h>
-
-#ifdef main
-#undef main
-#endif
-
-SDL_FORCE_INLINE void N3DS_Init(void);
-SDL_FORCE_INLINE void N3DS_Quit(void);
-
-int main(int argc, char *argv[])
-{
-    int result;
-    N3DS_Init();
-    SDL_SetMainReady();
-    result = SDL_main(argc, argv);
-    N3DS_Quit();
-    return result;
-}
-
-SDL_FORCE_INLINE void
-N3DS_Init(void)
-{
-    osSetSpeedupEnable(true);
-    romfsInit();
-}
-
-SDL_FORCE_INLINE void
-N3DS_Quit(void)
-{
-    romfsExit();
-}
-
-#endif /* __3DS__ */
 
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