Added FrameBegin/FrameEnd in RIBs generated by RenderManRender.

This relies on functionality introduced in ImageEngine/cortex/#329. If used without that Cortex functionality, it will simply result in a warning message stating that "ri:frame" is an unsupported option - so it's fairly harmless to merge this before the Cortex feature is deployed everywhere.

Fixes #358.
Merge pull request #971 from johnhaddon/riFrame

Added FrameBegin/FrameEnd in RIBs generated by RenderManRender.