Wii: Port to SDL2

[Ghabry]

There is now a SDL2 library for Wii and according to a discussion on gbatemp it is faster than the SDL1 library (better hardware renderer I guess).

Worth checking if it works properly, then we do not have to fix the semi-broken input stuff on the Wii with SDL1 :sweat_smile:

The AESND stuff can stay for performance reasons.

[Ghabry]

Made a quick port:

1. The image quality is much much sharper
2. It handles controller switching fine. A wiimote alone reports the dpad sideways and rotates the dpad when attaching a Nunchuck
3. USB keyboards work
4. The Wii IR works as a mouse cursor
5. Not crashing in Yume Nikki when switching maps compared to SDL1 but this is likely a bug in the aesnd backend

Negative:

1. Rendering is always in 4:3, but could be an issue on my side because the homebrew browser also renders this way
2. Compared to SDL1 performance is slightly slower (up to 5 FPS) when vsync is off (does not flicker). With vsync on performance is terrible.

I'm in favour of switching to it.