gamepad: Replace GetNumMappings/GetMappingByIndex with a single function.

Now it returns an array and optional count, to match other SDL3 APIs.

build-scripts/SDL_migration.cocci

@@ -1240,21 +1240,11 @@ typedef SDL_GameControllerButton, SDL_GamepadButton;
   (...)
 @@
 @@
-- SDL_GameControllerMappingForIndex
-+ SDL_GetGamepadMappingForIndex
-  (...)
-@@
-@@
 - SDL_GameControllerName
 + SDL_GetGamepadName
   (...)
 @@
 @@
-- SDL_GameControllerNumMappings
-+ SDL_GetNumGamepadMappings
-  (...)
-@@
-@@
 - SDL_GameControllerOpen
 + SDL_OpenGamepad
   (...)