The skip_frames flag was a non-configurable runtime flag. Since it is not exposed to the user, there is no need for a (possible) runtime cost. For testing purpose, we still want it to be configurable, so make it a compilation flag.
Only include SDL_stdinc.h for SDL_bool, not the whole SDL.h.
The required headers are included anyway.
Many functions returned an int to indicate their success. For clarity, use SDL_bool instead.
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