Prevent compilation warning about redefining SDL_HINT_RENDER_METAL_PREFER_LOW_POWER_DEVICE

[Lundis]

I'm compiling with dynamic SDL 2.28.3, on Ubuntu 23.10. Every time I compile my game, I get ~6 sets of warnings about SDL_HINT_RENDER_METAL_PREFER_LOW_POWER_DEVICE already being defined:

In file included from system.c:1:
hints.go:11: warning: "SDL_HINT_RENDER_METAL_PREFER_LOW_POWER_DEVICE" redefined
   11 | #define SDL_HINT_RENDER_METAL_PREFER_LOW_POWER_DEVICE ""
      | 
In file included from /usr/include/SDL2/SDL.h:47,
                 from /home/lundis/go/pkg/mod/github.com/veandco/go-sdl2@v0.5.0-alpha.6/sdl/sdl_wrapper.h:5,
                 from events.go:4:
/usr/include/SDL2/SDL_hints.h:1486: note: this is the location of the previous definition
 1486 | #define SDL_HINT_RENDER_METAL_PREFER_LOW_POWER_DEVICE "SDL_RENDER_METAL_PREFER_LOW_POWER_DEVICE"

So to avoid this spam, this PR undefines it first if it's defined already. The other new constants do not suffer from the same problem (on my setup, at least).

[Lundis]

Nevermind, the problem is actually that the function was added in 2.28, not 2.30. I'll close this and suggest the correct fix instead.