To "Support GPU hardware in headless mode"(765284), works need to be done on both WebGL part and compositor (VIZ) part. For the compositor part, I think this can be done in two steps:
1), Replace SoftwareRenderer with SkiaRenderer under headlesss mode. I think this is also a good case to verify if SkiaRenderer works well under software mode, so that we are confident in removing SoftwareRenderer finally. 
2), Enable the  GPU path on SkiaRenderer under headlesss mode.

Xing willing to help on this.