Blender Git Loki

Fix potential bad behavior, and cleanup/refactor a bit BKE_mesh_ensure_normals_for_display().

This is merely making behaviors of this function a bit more explicit,
and avoid re-adding another CD_NORMAL layer to polys in the (unlikely)
case it would already have one.

It also handles CD_MASK_NORMAL in cd_dirty_poly, but this is more like
future-proof thing, this is not used anywhere currently afaik.