Allow DCC brightness value offset for monitor

[mwhrtin]

I've got two monitors of the same brand and model that I've color calibrated to have matching color and brightness. To have them look identical I need to adjust the brightness on one of them to "+2" compared to the other, with equal brightness there's a small but noticeable difference.

I'm using the group feature to synchronize the DCC brightness adjustment which works great, except that I don't want them to be entirely equal. I want the brightness change to be synchronized, but I need it to still respect the difference in values that I originally had.

So I suggest a new "DCC brightness value offset" property where I can set +/- how much the brightness value should offset. E.g. if the offset is set to "+2" and the DCC brightness value is set to 50%, the monitor's actual brightness is set to 52%, and same if the offset is set to "-2", then it would be set to 48% instead.

[waydabber]

Right now you can simply alter the DDC ranges of the displays (however that will prevent you from attaining full brightness) - by limiting the range of the brighter one. You can do this under DDC Features.../Brightness/DDC value range maximum for the display.

I plan to add a proper brightness matching/normalizing feature for synchronization:

#1393