This is needed for compositors that want to reserve space for XWayland panels. Such a feature can be useful in a "transitional" setup, where only the X11 window manager and compositor is replaced but other components of an X11 desktop environment are still used.
This change simply reads the X11 property; the compositor is free to ignore it. Thus, compositors that don't want to support such a "transitional" feature are not impacted.
I can eliminate the xcb-ewmh dependency if that's preferred; it
would just require duplicating the xcb_ewmh_wm_strut_partial_t
struct.
Example compositor usage: https://github.com/labwc/labwc/pull/625