Auto-adjusting of poster to given t

app/assets/javascripts/player.js

@@ -8,7 +8,7 @@ function tryCreatePlayer(parentNode, asciicast, options) {
       {
         width: asciicast.width,
         height: asciicast.height,
-        snapshot: asciicast.snapshot,
+        poster: options.poster || asciicast.snapshot,
         speed: options.speed,
         autoPlay: options.autoPlay,
         loop: options.loop,

app/models/embed_playback_options.rb

@@ -1,3 +1,12 @@
 class EmbedPlaybackOptions < PlaybackOptions
   attribute :autoplay, Boolean, default: false
+
+  def as_json(*)
+    if !poster && t && t > 0
+      super.merge({ poster: "npt:#{t}" })
+    else
+      super
+    end
+  end
+
 end

app/models/playback_options.rb

@@ -22,6 +22,7 @@ class PlaybackOptions
   attribute :theme,     String,  default: Theme::DEFAULT
   attribute :t,         Time
   attribute :v0,        Boolean, default: false
+  attribute :poster,    String
 
   def as_json(*)
     opts = {
@@ -31,6 +32,7 @@ class PlaybackOptions
       preload: preload,
       fontSize: size,
       theme: theme,
+      poster: poster,
     }
 
     if t