Better icon for ditch/drain

[Kilkenni]

Current icons for waterway=ditch and waterway=drain differ very little (drain is slightly wider). At the default size, they are hard to distinguish for someone like me who doesn't use them daily.

Since the main feature differentiating a drain (wiki.openstreetmap.org/wiki/Tag:waterway%3Ddrain) in OSM is the lining, which a ditch does not have, change the color of the shape: brown for a ditch, current (or greyish like concrete) for a drain. Alternative: for the drain icon, replace the current lining with an imitation of bricks (maybe borrow these from the barrier=wall icon).