[0.10.1.2 beta release-584] Rivers appear empty in in-browser world map

[shardwolfdotnet]

After digging into #24688, we discovered that water was actually flowing correctly in the world (i.e. worldgen was not misbehaving - oops!), however the server's World Map (as viewed through the web interface) shows no water.

Consider the following two map snippets - the lower one is the same area of the world as seen through the in-browser world map:

Taking a sample of other servers, it appears that this impacts other servers as well - here's an example from another server's world map:

The in-game World Map renders these areas as-expected - the disparity is pretty confusing.