Add invert filter

[killerbee13]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the request will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.

Feature description

Add an "invert" or "negative" filter to the filters tab. This filter applies c = 255 - c to each channel of every pixel in the image.

Why do you want this feature?

Inverting images is a widely available filter in image editors that is useful as an aesthetic effect or occasionally for functional purposes. In particular I usually invert my avatars for my alt accounts, but forgot to prepare one on my PC for my newest avatar so I tried to do it on my phone and found I could not do it in this app.

Additional information

No response