Multi client jobs (#15)

* Add API to expose if RenderQueue is available to take new jobs for a given renderer and priority

* Fix issue with calculating Blender percent complete when not starting at 1

* Rename owner / client properties to parent / children

* Add make_ready method to API

* Create and submit subjobs to other servers

* Update make_ready to update children jobs and some misc fixes

* Misc GUI cleanup

lib/workers/ffmpeg_worker.py

@@ -8,10 +8,9 @@ class FFMPEGRenderWorker(BaseRenderWorker):
 
     engine = FFMPEG
 
-    def __init__(self, input_path, output_path, priority=2, args=None, owner=None,
-                      client=None, name=None):
+    def __init__(self, input_path, output_path, args=None, parent=None, name=None):
         super(FFMPEGRenderWorker, self).__init__(input_path=input_path, output_path=output_path, args=args,
-                                                 client=client, priority=priority, owner=owner, name=name)
+                                                 parent=parent, name=name)
 
         stream_info = subprocess.check_output([self.engine.renderer_path(), "-i",  # https://stackoverflow.com/a/61604105
                                                      input_path, "-map", "0:v:0", "-c", "copy", "-f", "null", "-y",