Additions/WDDM: add Retina-scaled video modes

[moreaki]

Summary

Add aspect-preserving intermediate modes to the Windows Guest Additions WDDM mode list.

For a 5760x3240 Retina target, the generated modes are:

• 5120x2880
• 3840x2160
• 3200x1800
• 2880x1620
• 2560x1440
• 1920x1080

Why

The previous built-in WDDM list exposed the native Retina target mode and then mostly legacy 4:3/5:4 modes such as 1920x1440, 1600x1200, and 1280x1024. That made Windows Display Settings awkward for high-DPI hosts because useful aspect-correct intermediate Retina modes were missing.

The new code derives scaled modes from the current/preferred target size and keeps the existing VRAM/frontend mode checks.

Validation

• Applied independently to origin/main.
• git diff --check passes.
• VBoxWddm build was attempted, but the local macOS ARM64 tree lacks a configured Windows SDK/WDK (PATH_SDK_WINSDK10), so the build stopped before compiling the driver.
• The mode-list behavior was validated through the existing driver's custom-mode registry path. Windows Display Settings exposed the same intermediate modes listed above.

Refs #742.

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