magickBrightnessContrastImage method
Use magickBrightnessContrastImage() to change the brightness and/or
contrast of an image.
It converts the brightness and contrast parameters into slope and
intercept and calls a polynomial function to apply to the image.
This method runs inside an isolate different from the main isolate.
brightness: the brightness percent (-100 .. 100).contrast: the contrast percent (-100 .. 100).
Implementation
Future<bool> magickBrightnessContrastImage({
  required double brightness,
  required double contrast,
}) async =>
    await _magickCompute(
      _magickBrightnessContrastImage,
      _MagickBrightnessContrastImageParams(
        _wandPtr.address,
        brightness,
        contrast,
      ),
    );