GUI: draw framebuffer updates without temporary pixmaps

[moreaki]

Summary

Draw framebuffer update rectangles directly from the backing QImage instead of constructing a temporary QImage view and converting it to a QPixmap for every painted rectangle.

The direct QPainter::drawImage call preserves the existing source rectangle, logical target position, and device-pixel-ratio scaling. The patch is confined to UIFrameBufferPrivate::drawImageRect; it does not alter Dock previews, opacity, background clearing, widget attributes, update scheduling, or platform policy.

Scope

This is a draft Qt frontend performance candidate, not a functional prerequisite for the Retina/HiDPI work 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.

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 with this patch in the integrated functional stack: pending after the final minimization.
• Static geometry audit: the source origin and clipped dimensions match the former sub-image; the target dimensions divide by the same device-pixel ratio formerly assigned to the temporary pixmap.
• Runtime correctness and isolated performance A/B at 3840x2160: pending.

Earlier measurements covered a larger combined patch and are not evidence for this minimized PR.

Refs #742.

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