SDL3: The "SDL_VIDEODRIVER" environment variable should be supported as a fallback

[DanielGibson]

Users are trained to use SDL_VIDEODRIVER=wayland for native Wayland support.
Renaming that env var to SDL_VIDEO_DRIVER breaks this.
As SDL3 is ABI-stable now it's probably too late to undo the renaming, but if the SDL_VIDEO_DRIVER env var isn't set it could (should!) still fall back to SDL_VIDEODRIVER.

Probably same for SDL_AUDIODRIVER/SDL_AUDIO_DRIVER.

I briefly looked at the code and am not sure how to best implement this: Hardcode the case in SDL_Get_Hint() and friends (if(result == NULL && strcmp(name, SDL_HINT_VIDEO_DRIVER) == 0) result = SDL_getenv("SDL_VIDEODRIVER");), or somehow handle this in the callers of SDL_GetHint(SDL_VIDEO_DRIVER)?
The latter is probably harder to do correctly if I want to respect hint priorities..

By the way, the (public) Hint API is missing a getter for a hints priority.

[DanielGibson]

Implementation of this: #11120

Not 100% sure if this is the best way to implement this, but I think doing this at all (before the first official) SDL3 release does make a lot of sense, and at least my solution is simple ;)