SceneNode : Improved bound propagation performance.
SceneNode : Improved bound propagation performance.

The `ScenePlug::bound()` and `ScenePlug::transform()` convenience methods both create a temporary context internally. We can therefore reduce the number of context creations by avoiding the convenience methods and making our own context once outside of the loop.

This shaves 7% off the time to propagate bounds when a Transform node is used with a particularly inefficient filter.

andrewkaufman/gaffer,hradec/gaffer,boberfly/gaffer,ivanimanishi/gaffer,chippey/gaffer,andrewkaufman/gaffer,hradec/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,chippey/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,chippey/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,hradec/gaffer,boberfly/gaffer,boberfly/gaffer,johnhaddon/gaffer,hradec/gaffer,chippey/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,boberfly/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,hradec/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,appleseedhq/gaffer