(release/25.0) Xephyr: fix setting physical output size

[metux]

Providing physical size via RRRegisterSize() is not enough - physical
size is a property of the output, not resolution and needs to be set
separately. Furhermore, get the physical output size using RandR if
-output is used, to match specific output, not the whole screen.

With this, Xephyr -output OUTPUT_NAME properly copies not only
resolution but also physical size of the named output.

Signed-off-by: Marek Marczykowski-Górecki marmarek@invisiblethingslab.com
Part-of: https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2240

[metux]

🤖 Automated review — generated by Starfleet ship Voyager on behalf of @metux. Not a human review.

Clean, well-scoped fix for Xephyr's -output option not propagating physical size.

The problem: RRRegisterSize() sets resolution but not physical size — physical size is a per-output property. When using -output OUTPUT_NAME, Xephyr copied the resolution but left the output's mm dimensions at defaults.

The fix:

1. Extends hostx_get_output_geometry() to return mm_width/mm_height from the RandR output info (single call site, consistent signature change).
2. Stores the physical size in new win_width_mm/win_height_mm fields on ephyrScrPriv.
3. In ephyrRandRGetInfo(), calls RROutputSetPhysicalSize() on the first output when in fullscreen mode — this is what actually makes the physical size visible to clients via RandR.
4. In fullscreen mode without -output, initializes from the host screen's millimeter dimensions. With -output, populated from the named output's RandR info.

ABI: Xephyr-only code (hw/kdrive/ephyr/). hostx_get_output_geometry and ephyrRandRGetInfo are not exported, not referenced by any NVIDIA blob. ABI-safe.

Looks good.