2D layering fixes

[Wumpf]

• fix layered images (same draw order) no longer being transparent
• fix z fighting in some cases
• improved draw order sample

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)

PR Build Summary: https://build.rerun.io/pr/2080

[teh-cmc]

Code looks fine but I was extremely surprised and confused by the fact that drawing two opaque images on the same plane suddenly resulted into blending? I've missed a discussion I guess 🤔

[Wumpf]

I've missed a discussion I guess

hmm not really. That's just "how it's always been". I think someone at some point pointed out that this behavior should be configurable in some way. But the behavior changing was unintentional and thus considered a regression