Change the behavior of AO pass in Blender internal's shader/render node tree so that it becomes (1.0, 1.0, 1.0) when AO is disabled.

For materials using AO pass, this makes the material preview and the GLSL
preview more accurate, but shouldn't affect final rendering in most cases
because we usually enable AO when using the AO pass in node tree.

Thanks to Brecht for code review.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61235 954f8c5b-7b00-dc11-b283-0030488c597c