Support for brightnesscontrastimage()

[BenMorel]

Currently, Gmagick provides support for contrastimage(), but not for brightnesscontrastimage().

The MagickBrightnessContrastImage() is here: https://www.imagemagick.org/api/MagickWand/magick-image_8c_source.html#l01053

Could you please add support for it? Thank you!

[boogiefromzk]

Yes, it would be useful.