Auto merge of #29939 - mrobinson:fix-float-placement-with-future-margins, r=mrobinson

Properly position floats when subsequent boxes collapse margins with containing block

Margins should be able to collapse through floats when collapsing with
parent blocks (the containing block). To properly place floats in this
situation, we need to look at these subsequent floats to find out how
much of the margin will collapse with the parent.

This initial implementation is very basic and the second step would be
to cache this in order to avoid having to constantly recalculate it.

Fixes #29915.

Co-authored-by: Oriol Brufau <obrufau@igalia.com>

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #29915.
- [x] There are tests for these changes

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