Fix deferred viewport repaint requests on Windows

[rustbasic]

This fixes an issue where deferred viewports may stop repainting after the root viewport receives focus on Windows.

When Windows does not deliver RedrawRequested events, expired repaint requests for deferred viewports are painted directly.

Each direct-paint pass uses a time budget to avoid delaying root-window event processing. To prevent consecutive event callbacks from repeatedly starting new direct-paint passes, each event-loop cycle allows at most one deferred direct-paint pass. Remaining deferred repaint requests are deferred to the next wake-up.

• 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

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/8467-p120
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