layout: Adjust inset-modified containing block to have non-negative size #56029
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If the insets of an absolutely positioned box are big enough that the inset-modified containing block (IMCB) would get a negative size, we will now adjust the weaker inset so that the size of the IMCB becomes zero instead.
This affects auto sizes, auto margins, and self-alignment.
This behavior matches Blink for the most part. However, while Blink also ensures a non-negative IMCB size, it doesn't necessarily do so by adjusting the weaker inset, so the alignment may differ.
Blink is fine with adopting this behavior, as resolved by the CSSWG in w3c/csswg-drafts#11478
Testing: Adding new WPT
Reviewed in servo/servo#40628