Improve rendering performance on Windows

[joordih]

Summary

Windows rendering has been noticeably behind macOS, especially menu and popup transitions. This PR closes most of that gap without changing a single pixel or animation: every change is Windows-gated, macOS and Linux are untouched.

What changed

Opaque main window on Windows (src-tauri/tauri.windows.conf.json, new)
The base config sets transparent: true for every platform, but only macOS uses it (the glass chrome is gated on platform-macos, and configure_app_window already paints the Windows background opaque). WebView2 pays a real cost for transparent windows: the whole surface goes through per-pixel alpha composition and loses GPU fast paths. Windows now gets its own platform config with transparent: false, mirroring how tauri.macos.conf.json already works. Since the window was always painted opaque there, nothing changes visually.

GPU rasterization flags for WebView2 (same file)
additionalBrowserArgs adds --enable-gpu-rasterization --enable-zero-copy --ignore-gpu-blocklist, with Tauri's default Windows args re-included since this field replaces them.

Compositor hints for popup transitions (use-platform-setup.ts, window-transparency.css)
The root element now gets a platform-windows class (Windows previously shared platform-other with Linux, so it couldn't be targeted separately). Menubar and combobox popups get will-change: opacity, transform, filter on Windows: the exact same open/close transitions, but composited on the GPU instead of re-rasterized on the main thread every frame.

Window controls and menu stacking (capabilities/main.json, menubar.tsx)
The custom title bar needs core:window:allow-minimize and core:window:allow-toggle-maximize to work on Windows. Menubar popup positioners get explicit z-index values so menus and submenus stack above the workbench chrome.

Notes

• No visual or animation changes on any platform.
• macOS and Linux code paths are byte-identical.
• tsc --noEmit clean.

[mehmetozguldev]

I appreciate the PR! I'll test the changes on Windows now and hopefully merge after that. Code looks clean

[mehmetozguldev]

Thanks again. I pushed a follow-up that keeps the opaque Windows window while removing production GPU override flags, makes platform classes exclusive, completes the window menu capabilities, and adds regression coverage.