Revert the the hue changes from invert

[klotzkoppp]

When not using grayscale, the invert filter not only changes brightness but also the hue of colors in the document. This can make colors in the document look weird, e.g. links are orange instead of blue.
Colors can be returned to their original hue by adding "hue-rotate(180deg)" to the invert filter if grayscale isn't used.

[waymondrang]

Implemented in #111 and will be included in the next release!