Better routing preview

[james1236]

• higher quality
• show buses
• show back and front at once
  • 50% transparent front when routing back
  • finish by swapping to front with back at 50%
• remain when finished
• sometimes no routing images are drawn
  • even though recieved clearly by front-end as seen in chrome network requests?
  • seems to happen when the board is below a certain size 🤔
• big front-end lag with routing images if the board is big