Let RiverWaterLevel respect UseWorldWaterLevel. Fixes #229.

Also fixed all water levels in a BiomeConfig being reset to the
WorldConfig values when turning on UseWorldWaterLevel.
Fix more errors in ReplacedBlocks

Fixes errors that could pop up in the following situations:

* WorldHeightScaleBits:7, WorldHeightCapBits:8
* WorldHeightCapBits:7
Fix mesa bryces

Two bugs were fixed:
* Fixed too many white stained clay appearing since
2f14f279c4a2403bc26ef4c52455ce540a9a92ec .
* Fixed bryce spikes generating in rivers when ImprovedRivers is set to
true. Fixes #250.