Version 0.22.0.0

This release brings support for dispatching via Pixar's Tractor, in
additional to the usual medley of bug fixes and optimisations.

Core

• Prevented the creation of cyclic connections (#1630).

UI

• Fixed bugs in "Set Key" plug menu item.
• Fixed Backdrop node positioning when creating a backdrop with no
  nodes selected (#1625).
• Fixed NodeEditor layout problems caused by long summaries (#1629).

Scene

• SceneProcedural (#1615)
  • Fixed bug which could cause motion blurred bounding
    boxes to be computed incorrectly.
  • Removed duplicate attribute computations. This shaves
    9% off the time to first pixel for a complex benchmark scene.

RenderMan

• Improved time to first pixel for raytraced 3delight renders (#1614).

Arnold

• Fixed warnings about inaccurate bounds (#1614).

Appleseed

• Added support for volume priority attribute (#1631).

Tractor

• Added a new GafferTractor module, which enables dispatching of
  Gaffer's task graphs to renderfarms running Pixar's Tractor (#1619).

Cortex

• Fixed UI for promoted presets parameters (#1624).
• Fixed parameter ordering in UI (#1627).

API

• ValuePlug
  • Prevented the addition of children which are not themselves
    ValuePlugs.
  • Allowed subclassing in Python.
• FilteredSceneProcessor
  • Removed restrictions on Filter::sceneAffectsMatch() (#1620).
• SceneProcedural/ScriptProcedural
  • Added support for using Renderer::Procedural::noBound (#1614).
• Fixed GIL management in DependencyNodeWrapper.
• Added DownstreamIterator.
• Improved error handling during dirty propagation.
• Batched dirty propagation during script loading, execution and
  destruction (#1632).

Tests

• Unexpected messages are now treated as errors.

Build

• Requires Cortex 9.6.0.