move webtorrent back into main window

We keep webtorrent running when the window is “closed” by hiding the
window instead.

main/lib/torrent-poster.js

@@ -0,0 +1,26 @@
+module.exports = torrentPoster
+
+var captureVideoFrame = require('./capture-video-frame')
+
+function torrentPoster (torrent, cb) {
+  if (torrent.ready) onReady()
+  else torrent.once('ready', onReady)
+
+  function onReady () {
+    // use largest file
+    var file = torrent.files.reduce(function (a, b) {
+      return a.length > b.length ? a : b
+    })
+    var video = document.createElement('video')
+    file.renderTo(video)
+
+    video.currentTime = 10
+    video.addEventListener('seeked', onSeeked)
+
+    function onSeeked (e) {
+      video.removeEventListener('seeked', onSeeked)
+      var buf = captureVideoFrame(video)
+      cb(null, buf)
+    }
+  }
+}