[performance] Avoid cross-viewport GalleyCache eviction

[rustbasic]

This changes Fonts to keep GalleyCache entries partitioned by viewport.

Previously, multiple viewports shared one galley cache. Since each viewport flushes the cache at the start of its pass, root and child viewports could repeatedly evict each other's text layouts.

As viewport text workload grows, this can force repeated layout of large text contents and cause significant performance degradation.

Font data and the texture atlas remain shared. Only layout caches are separated by an opaque viewport namespace.

• Closes Speed ​​issues on Windows when immediate viewport is on another screen #5249
• Related Resizable window jumps to incorrect size on trying to resize #8284
• Closes Severe performance degradation when rendering many Galleys in root or deferred viewports #8419

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/8418-p116
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

View snapshot changes at kitdiff