One correction to the fill generation;  otherwise, the top level
layout gets an ID layer dropped on it that's not supposed to be
there.

sky130/custom/scripts/generate_fill.py

diff --git a/sky130/custom/scripts/generate_fill.py b/sky130/custom/scripts/generate_fill.py
index 74b0c36..943920c 100755
--- a/sky130/custom/scripts/generate_fill.py
+++ b/sky130/custom/scripts/generate_fill.py
@@ -250,6 +250,7 @@
         print('gets $ifile xbase', file=ofile)
         print('gets $ifile ybase', file=ofile)
         print('close $ifile', file=ofile)
+        print('cif ostyle wafflefill(tiled)', file=ofile)
 
     # Now create simple "fake" views of all the tiles.
     print('gds readonly true', file=ofile)