Fix #34720: Sculpt mode T panel has no texture preview

Cycles was not able to render texture icons, same actually
goes to lux. Now made it so blender internal is used to render
icons for textures.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55499 954f8c5b-7b00-dc11-b283-0030488c597c