🖍🚀 Alternate position: fixed/absolute when docking/undocking (#32243)

- 🖍   Previously the video element would lag behind when using `position: fixed` at undock. `position: absolute` follows the video as it scrolls, eliminating the lag.  (This was backported from Bento version.)

- 🚀  Use existing `clientRect` where possible to prevent `getBoundingClientRect()` calls.

- 🚀  Migrates to `IntersectionObserver`.

Before / after (at 0.75 speed)
https://user-images.githubusercontent.com/254946/106031359-0756a300-6084-11eb-8bab-7cbe1f769313.mp4

build-system/test-configs/dep-check-config.js

@@ -361,10 +361,6 @@ exports.rules = [
         'src/service/position-observer/position-observer-impl.js',
       'extensions/amp-fx-collection/0.1/providers/fx-provider.js->' +
         'src/service/position-observer/position-observer-worker.js',
-      'extensions/amp-video-docking/0.1/amp-video-docking.js->' +
-        'src/service/position-observer/position-observer-impl.js',
-      'extensions/amp-video-docking/0.1/amp-video-docking.js->' +
-        'src/service/position-observer/position-observer-worker.js',
       'extensions/amp-analytics/0.1/cookie-writer.js->' +
         'src/service/cid-impl.js',
       'extensions/amp-next-page/0.1/next-page-service.js->' +

extensions/amp-video-docking/0.1/amp-video-docking.css

@@ -116,6 +116,7 @@
 
 .amp-video-docked-shadow {
   box-shadow: 0px 0 20px 6px rgba(0, 0, 0, 0.2);
+  transition-property: transform, opacity !important;
 }
 
 .amp-video-docked-controls-bg {
@@ -194,6 +195,9 @@
   transform-origin: left top !important;
 
   will-change: width, height, transition, transform, opacity;
+
+  /* Needs an arbitrary transition by default so browser won't glitch/jump. */
+  transition: 0.05s transform;
 }
 
 .i-amphtml-video-docked-overlay {