things have been feeling sluggish; let's try to gather data

163 columns, 919 panes:
  find on surface is slow
  changing font size seems to add sluggishness

I closed some single-pane columsn, now down to 95 columns, 793 panes.
Let's see how things go.

main.lua

@@ -310,6 +310,7 @@ end
 -- keep the structure of this function sync'd with refresh_pane_height
 function plan_draw(options)
 --?   print('update pane bounds')
+--?   print(#Surface, 'columns;', num_panes(), 'panes')
   Panes_to_draw = {}
   Column_headers_to_draw = {}
   local sx = Padding_horizontal + Margin_left
@@ -1639,3 +1640,11 @@ function to_text(s)
   end
   return Text_cache[s]
 end
+
+function num_panes()
+  local result = 0
+  for _,column in ipairs(Surface) do
+    result = result+#column
+  end
+  return result
+end