[BUG] Using Path Painter with Terrain Layer set causes inconsistent behaviour

[mrCurry90]

Describe the bug - Required
currently if you attempt to use the path painter while the terrain depth layer is set lower it creates the drawn path on the top of the terrain instead of applying it to the selected layer.

Steps To Reproduce - Required
Set Terrain Layer
Use Path Painter to paint underground

Expected behavior - Required
Path Painter should affect only visible layers

Attachments - Optional

Additional context - Optional
Reported on Steam by Spark_NV

[mrCurry90]

Actually a problem with all tools that apply terrain adjustment based current elevation.
Terrain above the currently set terrain layer can be affected, though in some scenarios this could be useful.
Better to let the user decide with a checkbox in each tool to toggle this behaviour on/off.