auto invert image behaviour

[amitu]

Since CSS in modern browsers can auto invert images we can use that for better dark mode support.

Since it does not make sense to invert all images, it has to be an opt in behaviour.

-- kernel ftd.image:
ftd.image-src src:
bool invert-on-light:
bool invert-on-dark:

Or better:

-- string enum inversion-behaviour:
invert-on-dark: invert-on-dark
invert-on-light: invert-on-light
do-nothing: do-nothing

-- kernel ftd.image:
ftd.image-src src:
inversion-behaviour inversion-behaviour:

We assume the provided image is light mode image, so we use the src.light as actual src and ignore src.dark, and apply the filter: invert(1) in dark mode.

And vice versa for invert-on-light.

Our current behaviour is do-nothing.

Since currently we do not have enum support, we will do the validation in Rust.

-- kernel ftd.image:
ftd.image-src src:
string  inversion-behaviour: