Fix: Make sea level locks aware of neighbouring canals

[zephyris]

Canal locks checked for elevation, to use alternate graphics with mini-lighthouses and breaking waves when at sea level. This looked odd when building locks on sea level land.

Therefore, add a canal_dike check, to check if canal walls would be built for that tile as a half-decent proxy for checking for neighbouring land tiles. Compare the top two locks in the example image.

(mostly) Fixes #237

Corner cases are when the lock is surrounded by sea level locks or rivers. Com[are the bottom two locks in the example image.

[LordAro]

I guess it's probably an improvement overall, but to me that left-hand lock will be more common than would be desirable. Have you had a look at it in "real world" maps ?

[zephyris]

Fair point. Though, lock next to canal in general doesn't look great. More of an issue of OpenTTD drawing the canal bank next to the lock?

[mmtunligit]

this seems to me like the proper solution would be making canals aware of locks, and not drawing that extension if there is one, i dunno if thats possible without code changes in the main repo though