undo creating new drawings

main.lua

@@ -219,11 +219,13 @@ function App.draw()
         button('draw', {x=4,y=y+4, w=12,h=12, color={1,1,0},
           icon = icon.insert_drawing,
           onpress1 = function()
-                       Drawing.before = snapshot()
+                       Drawing.before = snapshot(line_index-1, line_index)
                        table.insert(Lines, line_index, {mode='drawing', y=y, h=256/2, points={}, shapes={}, pending={}})
                        if Cursor1.line >= line_index then
                          Cursor1.line = Cursor1.line+1
                        end
+                       save_to_disk(Lines, Filename)
+                       record_undo_event({before=Drawing.before, after=snapshot(line_index-1, line_index+1)})
                      end})
           if Search_term == nil then
             if line_index == Cursor1.line then

undo.lua

@@ -42,6 +42,11 @@ function snapshot(s,e)
   elseif e == nil then
     e = s
   end
+  assert(#Lines > 0)
+  if s < 1 then s = 1 end
+  if s > #Lines then s = #Lines end
+  if e < 1 then e = 1 end
+  if e > #Lines then e = #Lines end
   -- compare with App.initialize_globals
   local event = {
     screen_top=deepcopy(Screen_top1),