N3DS port (squashed)

A dedicated renderer using Citro3D would likely allow for better
much better graphical performances.

include/SDL_platform.h

@@ -221,6 +221,11 @@
 #define __VITA__ 1
 #endif
 
+#if defined(__3DS__)
+#undef __3DS__
+#define __3DS__ 1
+#endif
+
 #include "begin_code.h"
 /* Set up for C function definitions, even when using C++ */
 #ifdef __cplusplus