add tone-mapping controls(gamma, exposure, white level) to bindings

[xieyizheng]

btw Great admirer of your works and beautiful software!

In my usage scenarios, I often find myself adjusting the gamma slider (usually increasing it) to get a rendering look that feels just right.

In order to programatically get the same look and fix it in code, now one could simply additionally tune the image after screenshotting, although that feels like a bit of a hack. It would be wonderful to add bindings for these controls to get a nice WYSIWYG flow.