When I lower ground too much, graphical glitches on earth border start to appear, caused by the fact that some areas are not painted properly. In function grund_t::display_border we are using world::get_groundwater() to determine minimal height, but that doesn't seems to change when digging. This patch replaces that with min_height
In in r11827. Thank you, I saw this for a long time but did not see the solution.