GUI: batch framebuffer update notifications

[moreaki]

Summary

Batch guest framebuffer update notifications in the Qt frontend before issuing viewport updates.

• Queue NotifyUpdate/NotifyUpdateImage rectangles in UIFrameBufferPrivate and schedule one GUI-thread flush while updates are pending.
• Flush pending rectangles in small bounded batches instead of posting one queued signal per rectangle.
• Reuse the existing UIMachineView dirty-rectangle conversion through a helper and mark the viewport as static/no-system-background so Qt avoids unnecessary background repaint work.

Why

This is a draft performance candidate. The macOS/Arm VMSVGA DX11 path can emit bursts of guest updates at HiDPI and 4K sizes; reducing queued signal churn and avoidable viewport background work may lower UI latency and active CPU, but it still needs isolated benchmark validation before it should be considered ready.

Validation

• git diff --check
• Benchmark validation pending with https://github.com/moreaki/dxmtbench and the existing Windows Explorer open/close workload across windowed, 4K, fullscreen, and Retina modes.

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