Add water tile support to autumn-outside heightmap

[XenonA7]

Water tiles are not a supported ground type when using the heightmap for the autumn-outside tileset. It would be very helpful if water was supported as it is in the bergen-trail and other tilesets.

For example, open autumn/path-06 and look at the heightmap.

When generating heights, it simply fills the river with grass.

It is evident that the devs also did not add heightmap support for water in autumn-outside (because the river uses the solid pattern heightmap tiles) but it would still be a useful feature.