Optimise CSS Grid

[nicoburns]

Objective

This targets a couple of obvious optisations of the grid algorithm:

• Only compute child intrinsic size when needed (previously it was computing these eagerly)
• Add early exit if only grid container measurement is required (similar to flexbox)
• Updates caching to cache width and height measurements separately. This isn't needed for flexbox, but makes a huge difference for grid.

It also updates the grid code to use inner_node_size (Size<f32>) rather than available_grid_space (Size<f32>) as inner_node_size should always be used for resolving percentages.

[alice-i-cecile]

These optimizations make sense to me. Do we have benchmarks for grid yet?

[nicoburns]

Do we have benchmarks for grid yet?

No, I was about to make some. But I got distracted doing the yoga ones 😂

[alice-i-cecile]

I'm happy to see this merged when you think it's ready :)