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 06d67b6abfd4d5ed8ceecd998b08cd1103946f26)

src/x11/meta-x11-selection-output-stream.c

@@ -110,7 +110,7 @@ get_max_request_size (MetaX11Display *display)
   if (size <= 0)
     size = XMaxRequestSize (display->xdisplay);
 
-  return size - 100;
+  return (size - 100) * 4;
 }
 
 static gboolean