assume starty can be nil in update

Dependencies

• [2] 3OTESDW6 move drawing.starty into line cache
• [*] BLWAYPKV extract a module
• [*] RT6EV6OP delegate update events to drawings

Change contents

• edit in drawing.lua at line 252
  [2.2924]

  [5.218]
  + if line_cache.starty == nil then

  + -- some event cleared starty just this frame

  + -- draw in this frame will soon set starty

  + -- just skip this frame

  + return

  + end