move webtorrent back into main window

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

main/lib/capture-video-frame.js

@@ -0,0 +1,23 @@
+module.exports = captureVideoFrame
+
+function captureVideoFrame (video, format) {
+  if (typeof video === 'string') video = document.querySelector(video)
+  if (!video || video.nodeName !== 'VIDEO') {
+    throw new Error('First argument must be a <video> element or selector')
+  }
+  if (format == null) format = 'png'
+  if (format !== 'png' && format !== 'jpg' && format !== 'webp') {
+    throw new Error('Second argument must be one of "png", "jpg", or "webp"')
+  }
+
+  var canvas = document.createElement('canvas')
+  canvas.width = video.videoWidth
+  canvas.height = video.videoHeight
+
+  canvas.getContext('2d').drawImage(video, 0, 0)
+
+  var dataUri = canvas.toDataURL('image/' + format)
+  var data = dataUri.split(',')[1]
+
+  return new Buffer(data, 'base64')
+}