bugfix: undo drawing creation

Also clean up drawing state to make sure we don't get into hard-to-debug
situations.

drawing.lua

@@ -348,7 +348,10 @@ function Drawing.mouse_released(x,y, button)
     end
   end
   save_to_disk(Lines, Filename)
-  record_undo_event({before=Drawing.before, after=snapshot_everything()})
+  if Drawing.before then
+    record_undo_event({before=Drawing.before, after=snapshot_everything()})
+    Drawing.before = nil
+  end
 end
 
 function Drawing.keychord_pressed(chord)

main.lua

@@ -142,6 +142,7 @@ 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_everything()
                        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