Improve Windows deferred viewport repainting with a redraw ledger

[rustbasic]

Summary

Builds on the Windows redraw-ledger approach proposed by MattReid87 to prevent deferred viewports from stopping repainting when the root viewport has focus.

On Windows, redraw requests are tracked in order. When a window receives a RedrawRequested event, it is painted first, followed by older outstanding requests in order. Direct paints are limited by a 5 ms soft time budget per event; requests not reached within that budget remain in the ledger. A paint that has already started may exceed the budget.

This keeps the ledger approach while limiting how long one redraw event spends catching up on deferred viewports. The change is Windows-only.

This PR supersedes #8467.

• Closes FontBook demo doesn't scale #4053
• Closes deferred viewport does not redraw when clicking ROOT viewport (Windows) #8351
• Closes Fix: deferred viewport does not redraw when clicking ROOT viewport (Windows) #8395
• Closes Deferred viewport does not redraw when the root viewport has focus (Windows) #8466
• Closes Fix deferred viewport repaint requests on Windows #8467

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/8650-p120-1
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[rustbasic]

I've been testing this extensively for a week, and it works great for me!