gpui: Add window composition tree

[huacnlee]

Objective

• Add a platform-independent window composition tree to GPUI.
• Support native and external GPU surfaces inside a GPUI window while preserving correct ordering, clipping, resizing, and GPUI overlays.
• Provide a working native WebView composition test on macOS and Windows.

Solution

• Add GPUI composition-tree types and APIs for base, overlay, native, and external GPU surfaces.
• Integrate composition geometry, scene splitting, deferred drawing, hit testing, window presentation, and surface lifecycle into Window and Scene.
• Add platform surface attachment hooks for bounds, visibility, parent-origin changes, compositor recreation, and platform handles.
• Implement macOS composition support using Metal-backed surfaces and native WKWebView embedding.
• Implement Windows composition support using DirectComposition visuals and per-surface swap chains, including:
  • GPUI base and overlay visuals;
  • native/external surface ordering and clipping;
  • resize and device-lost recovery;
  • compositor recreation callbacks;
  • visual opacity and visibility handling.
• Add the native_webview example with the existing macOS WebKit implementation and a Windows WebView2 ICoreWebView2CompositionController implementation.
• Forward mouse, button, modifier, and wheel input to WebView2 and update the WebView controller bounds during GPUI layout.
• Preserve GPUI popup menus, popovers, dialogs, and other overlay elements above the native WebView surface.
• Add Windows DirectComposition and WebView2 dependencies and document the example.

Linux and Wayland are not included in this change. They require a separate WebKitGTK/GTK integration and cannot reuse Windows DirectComposition directly.

Testing

• cargo check -p gpui --example native_webview
• cargo check -p gpui_windows
• Tested the Windows native WebView example with WebView2, including WebView interaction, resizing, GPUI popup menus, popovers, and dialogs.
• Tested the macOS composition example path through the existing WebKit implementation.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Improved GPUI window composition with native WebView and external surface support on macOS and Windows.