modifier stack: lazy initialize normals

many modifiers were calculating normals, when those normals were ignored by the next modifier.
now flag normals as dirty and recalculate for modifiers that set use `dependsOnNormals()` callback.

Quick test on mesh with 12 modifiers (mostly build type), calculated normals 6 times, now it only runs once - so this will give some speedup too.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57145 954f8c5b-7b00-dc11-b283-0030488c597c