This patch fixes a bug in GetImage on screens other than X11's screen 0.
This change is low-risk. The single-screen case is unchanged and the multi-screen case is almost certainly no worse than it was. Tested with xwd on a simulated two-screen system (XDarwin was hacked to report two screens even though there was only one monitor).
Should resolve #490586 "xwd in rootless mode with multi-monitors" and maybe #488868 "Xclearcase Kills XDarwin".