GUI: reduce macOS repaint and Dock preview overhead

[moreaki]

Summary

Reduce macOS Qt frontend repaint overhead for VM display updates.

The change:

• draws directly from QImage source rectangles instead of materializing a temporary QPixmap for each paint rectangle,
• throttles live Dock preview updates,
• avoids unnecessary full-surface backing-store fills on Qt 6/macOS.

Scope

This is a macOS frontend performance improvement, not part of the minimal functional Retina/HiDPI enablement tracked by #742.

The intended validation baseline for this PR is the functional/stability stack needed to run a Windows 11 ARM guest reliably on macOS/Arm with VMSVGA 3D and high-resolution modes enabled. It should be measured separately from #744 so the frontend repaint contribution is attributable.

Why

The macOS frontend was doing avoidable work for high-frequency guest display updates, which was especially visible with high-resolution Retina guest framebuffers.

Benchmark source

The reproducible benchmark suite used for final throughput validation is:

https://github.com/moreaki/dxmtbench

The suite runs browser-based WebGL/D3D-backed workloads inside the Windows guest, captures visual-output evidence, records throughput metrics, and exports machine-readable results. I will update this PR with isolated before/after dxmtbench results before treating it as a final performance submission.

Validation

• Applied independently to origin/main.
• git diff --check passes.
• Rebuilt locally as part of the macOS/Arm GUI test build.
• Runtime smoke-tested with a Windows 11 ARM guest at 5760x3240.

Refs #742.

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