Added option for group node buffering in the compositor.

Justa cluster did not have enough memory to handle all Mango 4k scenes.
Option is default disabled and can be enabled in the performance panel.

 - At Mind -



git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54599 954f8c5b-7b00-dc11-b283-0030488c597c