no, bring back that defense

Scenario where I ran into it:
  start a stroke inside a drawing
  press 'o' to switch to circle mode (but any mode switch will do)

The problem is that Text.textinput unconditionally blows away all .y
values. Sometimes we have to wait for a draw() to fill them back in.

drawing.lua

@@ -241,6 +241,10 @@ function Drawing.update(State)
   if State.lines.current_drawing == nil then return end
   local drawing = State.lines.current_drawing
   assert(drawing.mode == 'drawing')
+  if drawing.y == nil then
+    -- something invalidated all the y's; just wait for a draw
+    return
+  end
   local pmx, pmy = App.mouse_x(), App.mouse_y()
   local mx = Drawing.coord(pmx-State.left, State.width)
   local my = Drawing.coord(pmy-drawing.y, State.width)