[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.

src/xenia/helper/sdl/premake5.lua

@@ -0,0 +1,17 @@
+project_root = "../../../.."
+include(project_root.."/tools/build")
+
+group("src")
+project("xenia-helper-sdl")
+  uuid("84b00ad3-fba3-4561-96c9-1f9993b14c9c")
+  kind("StaticLib")
+  language("C++")
+  links({
+    "SDL2",
+  })
+  defines({
+  })
+  includedirs({
+  })
+  local_platform_files()
+  sdl2_include()