higher resolution for waterlevel

Interpret the waterlevel value as tenths of a cube.

This makes the waterlevel variable a float value to the user, while the
map file still stores an integer value - but now it is interpreted
differently - for example, a waterlevel value of 123 now means 12.3 cubes
instead of 123. Old map files are converted during loading.

Note: the values of waterlevel and hdr.waterlevel need to be in sync.

DOCREF waterlevel

source/src/entities.cpp

@@ -588,7 +588,7 @@ void flagdropped(int flag, float x, float y, float z)
     f.pos.x = round(p.o.x);
     f.pos.y = round(p.o.y);
     f.pos.z = round(p.o.z);
-    if(f.pos.z < hdr.waterlevel) f.pos.z = (short) hdr.waterlevel;
+    if(f.pos.z < waterlevel) f.pos.z = waterlevel;
     f.flagent->spawned = true;
     f.ack = true;
 }