Hold down Shift to embed points inside a mesh (or on a mesh, if there's no volume).
Done by gathering all the ray hits under mouse, sorting by depth and using the first two. Eventually could probably use hardware based depth peeling. Ray hit method could use bvh or some other accelerator too.
Eventually, will have a mechanism to keep consistent depth while painting continuous lines (painting over a partly obscured mesh shouldn't jump to the top one).
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