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
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