watercolor will now be used if it can be loaded

Right now the Biome Extractor does not extract this with the other
biome colors, so making it *required* would lead to a lot of errors
for people running servers that don't have the client jar.

Once the Biome Extractor has been changed, this should also.

overviewer_core/src/rendermodes.h

@@ -125,7 +125,7 @@ typedef struct {
     /* biome data for the region */
     PyObject *biome_data;
     /* grasscolor and foliagecolor lookup tables */
-    PyObject *grasscolor, *foliagecolor;
+    PyObject *grasscolor, *foliagecolor, *watercolor;
     /* biome-compatible grass/leaf textures */
     PyObject *grass_texture;