[css-grid-3][masonry] Make auto-fit do things

[fantasai]

As Tab pointed out in #9041 (comment), auto-fit repeat value is currently defined to behave identically to auto-fill in masonry. See https://drafts.csswg.org/css-grid-3/#repeat-auto-fit

repeat(auto-fit) behaves as repeat(auto-fill) when the other axis is a masonry axis.

AFAICT the rationale was that we can't redistribute space to the other tracks since placement depends on sizing in masonry layouts.

However, that doesn't mean we can't use the alignment properties to redistribute space, so I think we should allow auto-fit to drop columns just as is does in regular Grid layout, and just forbid it from looping back into the track sizing algorithm.

This means that, for example, if you have a masonry grid with 3 items but you could fit 6 columns, we'd drop the extra 3 columns and allow you to use justify-content: space-around to distribute the columns.