Adrian Wilkins-Caruana 0c9e8b3ece fix: Fix prewarm perf when a page contains many styles (#1451) ...

## Summary

**What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)

Fix prewarm perf when a page contains many styles.

The prewarm page buffer was a single slot, so each `prewarmCache` call
for a new font style freed the previous style's glyphs. On pages with
multiple styles (regular + bold + italic), only the last style was
prewarmed. The others fell through to the hot-group compaction path at
~2-3ms per glyph.

This was most visible on rich formatting (e.g. this [Czech prayer
book](https://stahuj.kancional.cz/e-kniha/kancional.epub) with bold
headings, italic liturgical text, and regular body), where page renders
took 3-5 seconds instead of ~700ms.

Fix: use up to 4 page buffer slots (one per font style) so all styles
stay prewarmed simultaneously.

Fixes #1450.

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2026-03-21 12:10:41 -05:00

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