Returns the sdl2:window that currently has input grab, or #f if no window has input grab. See SDL_GetGrabbedWindow.
NOTE: The returned window will be a new sdl2:window record pointing to the address of the existing window. It will be equal? to other sdl2:window records for the same window, but not eq?.
This procedure is available in sdl2 egg version 0.2.0 and higher. It requires SDL 2.0.4 or higher. It signals an error if the compiled version of SDL is not high enough. Use the libSDL-2.0.4+ feature identifier to check before calling this procedure.