Use this function to get the swap interval for the current OpenGL context.
Returns 0 if there is no vertical retrace synchronization, 1 if the buffer swap is synchronized with the vertical retrace, and -1 if late swaps happen immediately instead of waiting for the next retrace; call SDL_GetError() for more information.
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If the system can't determine the swap interval, or there isn't a valid current context, this function will return 0 as a safe default.
This function is available since SDL 2.0.0.