fix: posters flash between videos in playlist

[philjhale]

Description

When playing multiple videos in a playlist the videojs PosterImage will be hidden using CSS. However, the native poster attribute will briefly appear while the new source loads. This doesn’t look very nice. The fix hides the poster for every video after the first in the playlist. This is a compromise given it's a minor issue. The fix aims to cover the majority of cases. It does not cover all edge cases. E.g. Should posters be shown if there is a pause between playlist items? autoadvance > 0. Should the poster be hidden for the first video if the playlist has just repeated?

• Demo of the bug
• Demo of the fix

I'm very new to the videojs world so I'm very aware there's a lot I don't know. Please let me know what I've missed.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
• Reviewed by Two Core Contributors