N3DS port (squashed)

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

docs/README-n3ds.md

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+# Nintendo 3DS
+
+SDL port for the Nintendo 3DS [Homebrew toolchain](https://devkitpro.org/) contributed by:
+
+-   [Pierre Wendling](https://github.com/FtZPetruska)
+
+Credits to:
+
+-   The awesome people who ported SDL to other homebrew platforms.
+-   The Devkitpro team for making all the tools necessary to achieve this.
+
+## Building
+
+To build for the Nintendo 3DS, make sure you have devkitARM and cmake installed and run:
+
+```bash
+cmake -S. -Bbuild -DCMAKE_TOOLCHAIN_FILE="$DEVKITPRO/cmake/3DS.cmake" -DCMAKE_BUILD_TYPE=Release
+cmake --build build
+cmake --install build
+```
+
+## Notes
+
+-   Currently only software rendering is supported.
+-   Window are created on the top screen by default, use the `SDL_WINDOW_N3DS_BOTTOM` flag to put them on the bottom screen.
+-   SDL2main should be used to ensure all the necessary services are initialised.
+-   By default, the extra L2 cache and higher clock speeds of the New 2/3DS lineup are enabled. If you wish to turn it off, [use the PTMSYSM service](https://libctru.devkitpro.org/ptmsysm_8h.html#ae3a437bfd0de05fbc5ba9a460d148430) to turn it off in your program.