Add support for Gray8 images natively

[lambdachad]

Feature Description

Slint is missing some features that would make building software for Grayscale targets easier. It also helps to save memory and increase performance.

One feature that would be nice to have is support for Gray8 images. I implemented it here: lambdachad@ee2e0d5

Product Impact

Makes building products with Slint for E-INK screens easier, as it supports Grayscale better then.

[ogoffart]

That would make a lot of sense, yes.

[ogoffart]

It would be aswome if you could make a PR from your commit. I have a few comments to make :-)

[lambdachad]

Yes I'll go ahead and do that from latest.

[lambdachad]

Created pull request: #11331