Allow for painting with fixed Up direction (ignoring surface normal)

[kmcluckie]

Allows for painting objects like trees on hillsides, where the trees should still point directly upward.

The one issue this has is the added checkbox pushes the UI down and is starting to obscure the prefab grid. I was not sure how to change this, as I am quite new to Unity. Perhaps the maintainer could make this tweak? Sorry! My newness to Unity and 3D programming in general may also be evidenced by misuse of terminology ("up", "normals", etc).

I have also included a bunch of comments for the new code, which may seem verbose. I just love comments for my own use, but it doesn't really fit the current coding style, so can be stripped.