CS: Make painting terrain height over a cell border equalize the height at both edge

This MR change 2 things:

1. It modify the overly restrictive terrain height. In particular, it make it possible to edit a cell who have open edge on the cell, by making it ignore those (see #8683 ). It also change the function to limit only as much as is needed to be able to save the data. This, it will never refuse any edit (thought it might still auto-correct them, potentially creating an hole in the process, that the user would have to correct. That will only happen if the user does an edit that wouldn’t be saveable).
2. To compensate, as it won’t automatically link (small) hole anymore, the brushes has been altered to make it so each of the 2 edge/4 corners have the same height.

I found that the top column (the one in top) of a cell doesn’t get colored. That match with how it is in 49.0. I suspect something render-related.