Cycles: when render layers have per layer number of samples set, the Sampling
panel now has an option to specify how to use them. There's three options:

* Use: render layer samples override scene samples
* Bounded: bound render layer samples by scene samples
* Ignore: ignore render layer sample settings

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56096 954f8c5b-7b00-dc11-b283-0030488c597c