windows: Clip rounded retained layers

[BumpyClock]

Summary

• apply rounded retained-layer masks to the untransformed DirectComposition root visual
• keep compositor-driven child transforms and opacity inside the fixed destination-space clip
• clear the clip when returning to direct swap-chain presentation
• cover rectangular and asymmetric rounded mask conversion

Why

The rounded content-mask change added shader coverage for Windows primitives, but the DirectComposition retained-layer fast path bypassed the final rounded mask. A full-viewport retained layer could therefore move cached pixels into rounded-corner regions during compositor-only transforms.

Verification

• cargo check -p gpui
• cargo check -p gpui_windows
• cargo fmt --all --check
• git diff --check
• local CodeRabbit review: 0 findings
• independent Codex review: no blockers

The local x86_64-pc-windows-msvc cross-check reached dependency compilation but was blocked by the macOS host lacking lib.exe; Windows CI provides native validation.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: f117e07b-0e66-431a-8cf3-c3b631e40651

📥 Commits

Reviewing files that changed from the base of the PR and between 692d54d and 9f1fee1.

📒 Files selected for processing (2)

• crates/gpui_windows/src/directx_renderer.rs
• crates/gpui_windows/src/retained_compositor.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/windows-rounded-retained-layer-mask

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