fix(gui_building_grid_gl4): use correct water level value (#3030)

This fixes the grid display when map_waterlevel is set (which really
moves the map rather than the water, which stays at zero), and uses the
proper water level engine function for when the water level might
actually be non-zero in the future.

luaui/Widgets/gui_building_grid_gl4.lua

@@ -24,7 +24,7 @@ local config = {
 	lineColor = { 0.70, 1.0, 0.70 }, -- color of the lines
 }
 
-local waterLevel = Spring.GetModOptions().map_waterlevel
+local waterLevel = Spring.GetWaterPlaneLevel and Spring.GetWaterPlaneLevel() or 0
 
 local cmdShowForUnitDefID
 local isPregame = Spring.GetGameFrame() == 0 and not isSpec