KmlDataSource #9327 : handle relative URLs for video etc.
KmlDataSource #9327 : handle relative URLs for video etc.

Fix for KmlDataSource issue #9327 to handle relative URLs for additional elements like video etc.

CesiumGS/cesium,AnalyticalGraphicsInc/cesium,likangning93/cesium,likangning93/cesium,likangning93/cesium,CesiumGS/cesium,CesiumGS/cesium,CesiumGS/cesium,CesiumGS/cesium,AnalyticalGraphicsInc/cesium,likangning93/cesium,likangning93/cesium