wlroots reads these environment variables

wlroots specific
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* *WLR_DRM_DEVICES*: specifies the DRM devices (as a colon separated list) instead of auto probing them. The first existing device in this list is considered the primary DRM device.
* *WLR_DRM_NO_ATOMIC*: set to 1 to use legacy DRM interface instead of atomic mode setting
* *WLR_LIBINPUT_NO_DEVICES*: set to 1 to not fail without any input devices
* *WLR_WL_OUTPUTS*: when using the wayland backend specifies the number of outputs
* *WLR_X11_OUTPUTS*: when using the X11 backend specifies the number of outputs

rootston specific
------------------
* *XKB_DEFAULT_RULES*, *XKB_DEFAULT_MODEL*, *XKB_DEFAULT_LAYOUT*, *XKB_DEFAULT_VARIANT*, *XKB_DEFAULT_OPTIONS*: xkb setup

generic
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* *DISPLAY*: if set probe X11 backend in *wlr_backend_autocreate*
* *WAYLAND_DISPLAY*, *_WAYLAND_DISPLAY*, *WAYLAND_SOCKET*: if set probe Wayland backend in *wlr_backend_autocreate*
* *XCURSOR_PATH*: directory where xcursors are located
* *XDG_RUNTIME_DIR*: used to store non persistent temporary files

Document env vars used by wlroots
Document env vars used by wlroots
