eeshow/README: discuss use of *-cache.lib; TODO: areas enclose closed paths

eeshow/TODO

@@ -37,6 +37,9 @@ Rendering:
 - 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
+- diff area algorithm will fill anything resembling a closed path, no matter
+  how little is inside (e.g., a changed large but empty rectangle will be
+  filled)
 
 GUI:
 - fonts grow beyond their bounding boxes when zooming in