Profile-guided performance optimization for scrolling

[texodus]

This PR addresses Chrome-specific (though broadly applicable) performance/DOM/layout optimization, based on Chrome profiler output, specifically targetting scroll performance in Perspective (which has complex colspan and rowspan layout invalidations).

• Replaces continuous measure+apply column autosizing with ResizeObserver based sizing on the header only, which dramatically reduces forced relayouts reading offsetWidth.
• Memoize header cells (like body cells have always done).
• Use the predraw implementation previously added to render scroll frames also (using requestAnimationFrame to flush synchronously).
• Opt-in column_classes API so Perspective need not set style or class on all cells.
• Coalesce scroll events. I don't know if anyone uses these, but Perspective does not and they are profiler noise.

My totally non-scientific benchmark shows a 10ms ~> 2ms (per frame) improvement in non-JavaScript render time during scroll for a complex Perspective test case.