VMSVGA: coalesce DX11 readbacks and skip unchanged updates

[moreaki]

Summary

Reduce VMSVGA DX11 output readback and repaint overhead.

The change:

• coalesces pending output update rectangles,
• copies only the dirty rectangle from the screen texture to the staging texture,
• skips host-visible update sequence bumps when readback bytes are unchanged,
• adds STAM counters/profiles used for measurement.

Scope

This is a 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. That includes the functional HiDPI prerequisites from #742 and the runtime stability fixes needed to make the benchmark path reliable.

Why

On macOS/Arm with Windows 11 ARM and VMSVGA 3D enabled, simple desktop activity produced high host CPU usage. A large part of the cost came from repeated readback/frontend update work even when the visible output did not materially change.

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 VBoxDD/VirtualBoxVM graphics test build.
• Earlier manual Explorer open/close measurements showed a large improvement for the combined local patch stack, but those numbers are not claimed here as isolated proof for this PR.

Refs #742.

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