Feature request: Add RGB adjustment feature for gamma

[IceAsteroid]

Hi, I'm using this tool instead of others, because I have an urge to adjust contrast of the screen, while others don't have this functionality.

Everything's almost perfect, except the lack of feature for RGB adjustment while other tools all have this feature when it comes to gamma adjustment.

Could this feature be added?

I have no programming background, so I cannot push a Pull Request, I'm sorry.