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

include/SDL3/SDL_video.h

@@ -74,6 +74,9 @@ typedef enum SDL_SystemTheme
     SDL_SYSTEM_THEME_DARK       /**< Dark colored system theme */
 } SDL_SystemTheme;
 
+/* Internal display mode data */
+typedef struct SDL_DisplayModeData SDL_DisplayModeData;
+
 /**
  * The structure that defines a display mode.
  *
@@ -95,7 +98,9 @@ typedef struct SDL_DisplayMode
     float refresh_rate;             /**< refresh rate (or 0.0f for unspecified) */
     int refresh_rate_numerator;     /**< precise refresh rate numerator (or 0 for unspecified) */
     int refresh_rate_denominator;   /**< precise refresh rate denominator */
-    void *driverdata;               /**< driver-specific data, initialize to 0 */
+
+    SDL_DisplayModeData *internal;  /**< Private */
+
 } SDL_DisplayMode;
 
 /**