docs: Adding darkness and brightness examples

[erickzanardo]

Description

Adds two additional examples on the image stories for brightness and darkness changes on images.

https://user-images.githubusercontent.com/835641/229366147-df4701df-901c-4d50-827e-01b57c9f57f9.mov

Checklist

• [x] I have followed the Contributor Guide when preparing my PR.
• [x] I have updated/added tests for ALL new/updated/fixed functionality.
• [x] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [x] I have updated/added relevant examples in examples or docs.

Breaking Change?

• [ ] Yes, this PR is a breaking change.
• [x] No, this PR is not a breaking change.

Related Issues

[spydon]

The blue artifact is not there anymore, but it still produces a white rectangle on the transparent background.

[erickzanardo]

The blue artifact is not there anymore, but it still produces a white rectangle on the transparent background.

@spydon ins't the white rectangle expected? I remember we discussed something about it internally?

[spydon]

@spydon ins't the white rectangle expected? I remember we discussed something about it internally?

That was because previously flame.png had a black background, but it has a transparent background now and a fully transparent color shouldn't be brightened? Maybe that is actually a bug in our code?

[CodeDoctorDE]

Hi, I randomly found this pr here and I like the image example here. It would be nice if this could be merged so it can also be found easily on the website. What's missing? Can I help here?

[spydon]

Hi, I randomly found this pr here and I like the image example here. It would be nice if this could be merged so it can also be found easily on the website. What's missing? Can I help here?

As you can see in the video it wasn't working like intended. If you want to help you can checkout the PR, run the example and see if it works on the latest version of Flutter. :)