Multires baker: fix memory leak caused by threading issues

Didn't notice this before because of non-working guarded allocation
at the time threading was added to multires baker.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53969 954f8c5b-7b00-dc11-b283-0030488c597c
Multires baker: fix bad face->grid index conversion for displacement baker

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53970 954f8c5b-7b00-dc11-b283-0030488c597c
Multires baker: fix wrong normalization if baking happens to multiple images

Previously normalization will happen per image buffer individually, which
was wrong in cases different faces of the sane mesh uses different images.

Also solved possible threading issues when calculating min.max displacement.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53971 954f8c5b-7b00-dc11-b283-0030488c597c