Problematic logic on lower-than-sea pixels

[ahmetlii]

Steps to reproduce:

• bother --scale-data 0.5 --bounds 38.05 39.33 48.00 51.00 test.png --raise-undersea
• Scale the map and open in OpenTTD

As one might notice, passing --raise-undersea exacerbates the issue, most of the intended land is still below water. What happens if --raise-low is passed? The situation becomes even worse, not only that it doesn't work it causes huge rectangles over the sea to show up. After tinkering out a bit and looking at alternatives such as Tangram, I figured out that the idea behind --raise-undersea is just wrong. What should have been, is that the option would specify what is the threshold for minimum land level, which should have been around -28m for this use-case instead of 0, and should convert that to above sea level before scaling between 0-255. of course one might also say that using --lakes could solve that, but as far as I figured out it doesn't.