paintRiver cleanup.

Pass vectors, refs #4845.
Pass tile sizes rather than percent numbers of the map, refs #4939.
Replace some water- and land-functions with more common createArea+RectPlacer and paintTerrainBasedOnHeight calls.

Implement getMapBounds to make references easier to read, refs #4854 (and thus less error-prone, refs rP20465).
Move height constants to the top of the files.

Fix Aegean Sea, English Channel and Danubius fadeWidth off by factor of 2 in rP20185.
Fix forgotton clWater in unknown_common of rP20866 and move that createUnknownObjects call to common.