Light linking (#2875) :
Added a “defaultLights” set for use in light linking set expressions.
Added a “defaultLight” plug to all Light nodes, to control membership in the “defaultLights” set.
Significantly optimised the processing of light links in Gaffer and in the Arnold renderer backend.
Viewer : “userDefault” metadata can now be used to configure the default settings for Views (#2893).
PythonCommand/SystemCommand : Empty commands are now ignored (#2896).
SceneWriter : Added support for subclassing in Python (#2901).
Checkerboard : Improved performance (#2912).
SceneInspector : Fixed bug which caused sections to be disabled (#2903).
Layout Menu : Fixed clashes between custom layout names and standard menu items. For instance, previously you could save a layout called “Delete” and it would mean that the standard “Delete” submenu was no longer available (#2899).
Grade : Fixed bug that caused clamping to be ignored if the other settings were at default values (#2908).
CompoundDataPlug/ValuePlug : Fixed GIL management bugs which could cause Gaffer to hang (#2907).