Parallel operator pipelines

Allow both GPU and CPU parallel pipelines. Currently multiple GPU streams are used if framesets are given as inputs. However, parallelism could be performed on a single frame to improve GPU utilisation (marginally) and allow CPU processing in parallel.

Current ideas:

  1. Add a wait method to operators
  2. Add a special parallel operator to which other pipelines can be attached, using the wait method to merge back
Edited by Nicolas Pope