Fix visible seams on normal map bake due to clear colour being black:

 This is a special case, tangent space normal maps should be cleared to
0.5, 0.5, 1.0. This is good practice but there's no reason why not to
automate this (for artists whose bump map fu may be a bit shallow).

Thanks to Morten Mikkelsen for reporting.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54693 954f8c5b-7b00-dc11-b283-0030488c597c