Wrong pixelAspectRatio population in DASH (#2614)

The current implementation of pixelAspectRatio in DASH is wrong. `@par` in DASH is "picture  aspect ratio" so the correct is populate this field with `@sar`.

Backported to v2.5.x

Change-Id: I5e492e17cee3895af43782b6799e09baa31cffcb

lib/dash/dash_parser.js

@@ -1438,7 +1438,7 @@ shaka.dash.DashParser.prototype.createFrame_ = function(
   let frameRate =
       XmlUtils.parseAttr(elem, 'frameRate', evalDivision) || parent.frameRate;
   const pixelAspectRatio =
-      elem.getAttribute('par') || parent.pixelAspectRatio;
+      elem.getAttribute('sar') || parent.pixelAspectRatio;
   let emsgSchemeIdUris = this.emsgSchemeIdUris_(
       XmlUtils.findChildren(elem, 'InbandEventStream'),
       parent.emsgSchemeIdUris);