Renamed driverdata to internal

This was done to SDL_DisplayMode for consistency with SDL_Surface and gives it a type so we don't have to do casts in SDL code.

I considered switching to an ID and hashing the driver data, etc. but all of that involved a lot of internal code churn and this solution gives us flexibility in how we handle this in the future.

After consideration, I made this renaming global across the project, for consistency.

Fixes https://github.com/libsdl-org/SDL/issues/10198

src/video/winrt/SDL_winrtpointerinput.cpp

@@ -57,7 +57,7 @@ WINRT_TransformCursorPosition(SDL_Window *window,
         return rawPosition;
     }
 
-    SDL_WindowData *windowData = window->driverdata;
+    SDL_WindowData *windowData = window->internal;
     if (windowData->coreWindow == nullptr) {
         // For some reason, the window isn't associated with a CoreWindow.
         // This might end up being the case as XAML support is extended.