x11: XMaxRequestSize returns 4-byte units

XMaxRequestSize/XMaxExtendedRequestSize are documented to return
the maximum size in 4-byte units, whereas we are comparing this
to byte lenghts. We can afford 4x the data here.

Since I don't know the payload size of the XChangeProperty request,
be generous and allot 400 bytes for it, we have some to spare.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1198

(cherry picked from commit 06d67b6a)