Fix #37045: blender internal SSS rendering black on some system when the SSS
reflection color was set to 1 1 1. BSSRDF parameter fit not converging properly
due to float precision issues.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60708 954f8c5b-7b00-dc11-b283-0030488c597c