Added FnSceneShape createChild, extracted from expandOnce.
Fixed IECoreGL::PointsPrimitive crash on OS X.

It seems that some drivers don't like being passed 0 points to be rendered.
Merge pull request #149 from johnhaddon/zeroLengthGLPointsFix

Fixed IECoreGL::PointsPrimitive crash on OS X.
Merge pull request #148 from valeriebernard/fnSceneShapeCreateChild

Added FnSceneShape createChild, extracted from expandOnce.