[css-grid-3][masonry] Prioritize tracks that don't create overflow

[fantasai]

Pulling out @ethanjv's comment:

Btw we might want to consider that, in the current draft of masonry, having definite tracks mixed with intrinsic tracks might result in some items overflowing the spanning tracks in the grid axis. E.g., if we have a grid-template-columns: 50px auto definition, if we have two auto placed items, the first one with a min-content size of 100px and the other with 50px, the first will be placed in the first column and overflow it, then the second would be placed in the auto track.

Maybe we want to prioritize placing a grid item in a track that won't overflow?

[fantasai]

There's two ways to address this use case:

• Give the author the ability to select a set of allowable tracks for auto-placement (rather than all or none, as currently).
• Have the UA skip over tracks that overflow. (This will be more expensive because you may need to test-fit into different tracks.)