VMSVGA: coalesce DX11 readbacks and skip unchanged updates

[moreaki]

Summary

Reduce VMSVGA DX11 screen-output readback work in three related places:

• Merge a newly queued screen rectangle into a compatible pending rectangle for the current readback fence.
• Copy only the union of the current fence's dirty rectangles from the B8G8R8X8 screen texture into its staging texture.
• Compare the mapped B8G8R8X8 rows with the host buffer and suppress the output-target sequence increment and host screen update when no bytes changed.

The I420 conversion/readback path remains full-frame and always reports changed output.

Scope

This is a draft performance candidate, not part of the minimal functional Retina/HiDPI enablement tracked by #742. It should remain draft—and should be closed if an isolated current-main A/B comparison does not show a repeatable benefit.

The patch intentionally keeps the existing readback-fence ordering, fixed eight-entry update queue, output-target conversion, and vmsvgaR3UpdateScreen delivery path.

Benchmark source

The validation tool is the fixed dxmtbench functional/performance harness. Performance samples are admissible only after the same run passes its browser framebuffer, guest screenshot, host presentation/signature, launch, and graphics-log gates.

Validation

• git diff --check: passed.
• Full clean Darwin/Arm64 build from current origin/main at this minimized patch tip: pending in the pinned comparison build.
• Static fence audit: rectangles already covered by an in-flight readback are not merged with new work; only entries tagged with the current readback fence are eligible.
• Static format audit: B8G8R8X8 uses bounded partial copies and byte comparisons; I420 preserves the original full conversion/copy behavior.
• Runtime correctness and isolated performance A/B at 3840x2160: pending.

Earlier manual measurements covered a combined local patch stack and are not evidence for this PR.

Refs #742.

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