Fix #36302: Multires baking to zero 0 was showing error but still was baking

We do not support baking to level 0, disabled this completely now.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@58752 954f8c5b-7b00-dc11-b283-0030488c597c
Use bool instead of int.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@58753 954f8c5b-7b00-dc11-b283-0030488c597c