GUI: request Retina backing size in macOS fullscreen

[moreaki]

Summary

Use the Retina backing-pixel size when macOS fullscreen autoresize requests a guest mode.

For a Retina fullscreen view that looks like 2880x1620 on the host, the GUI now requests the 5760x3240 guest framebuffer mode instead of the logical host size.

Why

The previous fullscreen/autoresize behavior did not reliably request true Retina guest modes on macOS. That made Windows guests look scaled and prevented practical use of native Retina backing resolutions.

Validation

• Applied independently to origin/main.
• git diff --check passes.
• Rebuilt locally as part of the macOS/Arm GUI test build.
• Runtime verified with Windows 11 ARM at 5760x3240x32.
• Verified Guest Additions run level 3 after entering the Retina guest mode.

Refs #742.

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