Marker Tracks cause exceptions when provided to IMPBuilder

[davidt-netflix]

Building an IMP with a Marker track does not appear to be currently supported in Photon. But the API does not prevent this, and there are no early validations that check for this. As a result, providing a Marker track to IMPBuilder.buildIMP_2013 causes a casting exception.

• Calls to buildIMP_2013 or buildIMP_2016 will all eventually end up at ImpBuilder.buildEDLForVirtualTracks.
• As a parameter, this method takes a list of VirtualTrack.
• However, this method immediately casts each VirtualTrack to IMFEssenceComponentVirtualTrack

• This causes an exception when one of the VirtualTrack is actually a IMFMarkerVirtualTrack.

  public static Map<UUID, List<Node>> buildEDLForVirtualTracks (Map<UUID, IMPBuilder.IMFTrackFileMetadata> imfTrackFileMetadataMap, List<? extends Composition.VirtualTrack> virtualTrackList, IMFErrorLogger imfErrorLogger) throws IOException, ParserConfigurationException{
  Map<UUID, List<Node>> imfEssenceDescriptorMap = new HashMap<>();
  
  for(Composition.VirtualTrack virtualTrack : virtualTrackList) {
      Set<UUID> trackResourceIds = IMFEssenceComponentVirtualTrack.class.cast(virtualTrack).getTrackResourceIds();

The tests in IMPBuilderFunctionalTest.impBuilderTest specifically only test Essence tracks, so this is never caught.

At minimum, Photon should be updated to fail more gracefully, either ignoring Marker tracks, or providing a warning earlier in the code that Markers are not supported. Ideally, this would be a good opportunity to add support for Marker tracks.