Quenty's Part to Terrain [Preserve old terrain]

Converts part to terrain. Unlike other Roblox part to terrain tools, this can keep existing terrain instead of overriding it. Note this is a bit slower, but has the benefit of not wiping out old terrain. Handles balls and blocks right now. This can be used to easily fill in water by turning off "overwrite terrain." You can press and hold ALT just like the terrain tools to select the terrain type you want. Press C to convert the selected part. You can keep the selected part if you want.