[FacuM]

Summary

• Add a RandR 1.5 setmonitor virtual monitor on an enlarged screen, with a no-op CRTC panning so Xorg does not clamp the pointer out of it. The layout is re-applied against layout daemons and restored on exit.
• Capture the screen with MIT-SHM/GetImage, paced by XDamage, and send the cursor as XFixes metadata.
• Map touch and pen through the XInput2 Coordinate Transformation Matrix, re-applied on hierarchy and screen changes.
• Detect X11 sessions and add EZD_LINUX_BACKEND=x11.
• Add gated tests run against Xorg with the dummy driver (tools/x11-dummy-test.sh), and run them in CI.

Testing

• cargo fmt --check, clippy -D warnings and cargo test --workspace all pass.
• tools/x11-dummy-test.sh passes (22 tests) on Xorg + xf86-video-dummy.
• End-to-end on dummy Xorg with i3: 120 frames over NVENC, 0 dropped, cursor metadata OK, layout restored.
• With xfwm4 compositor + xfsettingsd: no layout fights.

Not yet verified

• Real input hotplug with pen/touch on a physical X session with the tablet.
• Plasma X11 and GNOME on Xorg (needs an Ubuntu/Kubuntu 24.04 VM; Mutter may drop foreign monitors).
• The proprietary NVIDIA X driver.

Scope notes

• This PR is stacked on Add Sway and Hyprland backends to the Linux host #2 (wlroots), which is stacked on Add a GNOME backend to the Linux host #1 (GNOME). Review the X11 commit only; the earlier commits belong to those PRs.