Add 'Media stream with video and synchronized graphics' use case

This replaces the 'Media analysis visualization' use case, which
covered much of the same requirements.

index.html

@@ -393,14 +393,32 @@ <h3>Synchronized map animations</h3>
         </p>
       </section>
       <section>
-        <h3>Media analysis visualization</h3>
-        <p>
-          A video image analysis system processes a media stream to detect and
-          recognize objects shown in the video. This system generates metadata
-          describing the objects, including timestamps that describe when
-          the objects are visible, together with position information (e.g.,
-          bounding boxes). A web application then uses this timed metadata
-          to overlay labels and annotations on the video using HTML and CSS.
+        <h3>Media stream with video and synchronized graphics</h3>
+        <p>
+          A content provider wants to provide synchronized graphical elements
+          that may be rendered next to or on top of a video.
+        </p>
+        <p>
+          For example, in a talk show this could be a banner, shown in the lower
+          third of the video, that displays the name of the guest. In a sports
+          event, the graphics could show the latest lap times or current score,
+          or highlight the location of the current active player. It could even
+          be a full-screen overlay, to blend from one part of the program to
+          another.
+        </p>
+        <p>
+          The graphical elements are described in a stream or file containing
+          <a>media timed events</a> for start and end time of each graphical
+          element, similar to a subtitle stream or file. A graphic renderer
+          takes this data as input and renders it on top of the video image
+          according to the <a>media timed events</a>.
+        </p>
+        <p>
+          The purpose of rendering the graphical elements on the client device,
+          rather than rendering them directly into the video image, is to allow
+          the graphics to be optimized for the device's display parameters,
+          such as aspect ratio and orientation. Another use case is adapting
+          to user preferences, for localization or to improve accessibility.
         </p>
         <p>
           This use case requires frame accurate synchronization of the content