[SDL] Add global helper to setup the library.

Call to SDLHelper::Prepare() is needed before first SDL_InitSubSystem().
- Sets hints (SDL configuration vars).
- Configures logging.

premake5.lua

@@ -243,6 +243,7 @@ solution("xenia")
   include("src/xenia/gpu/null")
   include("src/xenia/gpu/vk")
   include("src/xenia/gpu/vulkan")
+  include("src/xenia/helper/sdl")
   include("src/xenia/hid")
   include("src/xenia/hid/nop")
   include("src/xenia/hid/sdl")