hints: Add "metal" to the list of SDL_RENDER_DRIVER hints.

include/SDL_hints.h

@@ -76,6 +76,7 @@ extern "C" {
  *    "opengl"
  *    "opengles2"
  *    "opengles"
+ *    "metal"
  *    "software"
  *
  *  The default varies by platform, but it's the first one in the list that