eeshow/gfx/diff.c (merge_coord): typo caused offset troubles

eeshow/TODO

@@ -37,6 +37,8 @@ Rendering:
 - return indication of whether diff found any differences
 - in diff, pass only options understood by cairo_png
 - find out why in Cairo PDF non-dashed lines disappear if CAIRO_LINE_CAP_ROUND
+- non-interactive diff mode currently requires both sheets to have identical
+  size
 
 GUI:
 - fonts grow beyond their bounding boxes when zooming in
@@ -67,7 +69,3 @@ GUI/glabels:
 - maybe indicate how it is used
 - there can be "hanging" pop-ups, see comment in gui/glabel.c:hover_glabel
 - should there be a way to remove glabel highlighting ?
-- comparing Neo900 fbc3a95 and 6a9f71a, sheet 2 has large offset differences,
-  causing hover to have displaced AoIs in diff_old, and both AoIs and overlays
-  are wrong in diff_new. Offset differences also defeat rapid switching between
-  old and new as a means to spot differences.