Saves joystick calibration data.
After all the headache of calibrating the joystick, you may not want to
make your poor users repeat the process every time they run your program.
Call this function to save the joystick calibration data into the
specified configuration file, from which it can later be read by
load_joystick_data(). Pass a NULL filename to write the data to the
currently selected configuration file.
Returns zero on success, non-zero if the data could not be saved.
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