[Feature] Invert Colors of Exporting Stills

[mezhgano]

Sometimes redactors ask to send them credits as still images or pdf with inverted colors, because it is easier them to read black text on white background and catch typos and etc...
I know we can adjust background and styles to achieve that, but it is much more hassle than just a simple checkbox in Delivery Page, like that:

I think checkbox might be visible only when "Whole Page Stills" output format is selected.

[LoadingByte]

This is a great idea! I'll definitely add it to the next version.