Add Post-Processing options

[KryptoSynth]

Currently bright images set as wallpaper cause my status bar (which is transparent and has white text) to be illegible:

I'd like to have an option added that will selectively darken bright images to a configurable degree to help make the home screen/lock screen more legible.

Maybe even some general post-processing options like these, found in Wallpaper Changer for Reddit.

[sbellon]

Or like Muzei does it:

I.e. different post-processing settings for home screen and lock screen, so you can have the same image there, but blurred, darker and a bit less colour on the lock screen and when unlocking the device, the image becomes clear.