Added rsl implementations of the compositing defined in IECore/CompositeAlgo.h.
Added rsl implementations of the compositing defined in IECore/CompositeAlgo.h.

code-google-com/cortex-vfx,code-google-com/cortex-vfx,code-google-com/cortex-vfx,code-google-com/cortex-vfx,ruschelp/cortex-vfx,ruschelp/cortex-vfx,ruschelp/cortex-vfx