[css-grid] Sizing algorithm for span >= 2 should be a generalization of span 1 #3616

I don't understand why there is a track sizing algorithm using items with a span of 1 (and intrinsic minimum and non-flexible maximum), and another one for items with a span >= 2 (not spanning tracks with flexible maximums).

I think both cases should use the same algorithm, just with the extra space distribution being more trivial to perform for items with a span of 1. However, they aren't exactly the same:

• With span >= 2, tracks with intrinsic minimums end up at least as big as the minimum contribution. For span 1, this is only ensured for auto minimums.
• With span >= 2, an auto minimum under a sizing min/max-content constraint behaves as min/max-content. With span 1 it's not exactly like this, because the value is later clamped between the minimum contribution and the max track sizing function (or fit-content argument).

Why the differences?