Auto merge of #29859 - mrobinson:float-negative-block-margin, r=Loirooriol

Layout 2020: Properly handle negative margins in floats

If a float has negative block margins, it should be pushed upward, but shouldn't affect the positioning of any floats that came before it. It should lower the ceiling though when it still has some non-negative block contribution. In order to implement this behavior, we should only place the float considering its non-negative block length contribution. If the float is pushed up completely past it's "natural" position, it should be placed like a float with zero block size.

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