biomes are now murky *and* smooth

closes issue #708

Props to mkor for taking advantage of the existing average loop, and
CounterPillow for sussing out the correct biome multiplication
color. This commit is just a teensy bit more future-proof.

overviewer_core/src/overviewer.h

@@ -26,13 +26,18 @@
 
 // increment this value if you've made a change to the c extesion
 // and want to force users to rebuild
-#define OVERVIEWER_EXTENSION_VERSION 31
+#define OVERVIEWER_EXTENSION_VERSION 32
 
 /* Python PIL, and numpy headers */
 #include <Python.h>
 #include <Imaging.h>
 #include <numpy/arrayobject.h>
 
+/* like (a * b + 127) / 255), but much faster on most platforms
+   from PIL's _imaging.c */
+#define MULDIV255(a, b, tmp)								\
+	(tmp = (a) * (b) + 128, ((((tmp) >> 8) + (tmp)) >> 8))
+
 /* macro for getting a value out of various numpy arrays the 3D arrays have
    interesting, swizzled coordinates because minecraft (anvil) stores blocks
    in y/z/x order for 3D, z/x order for 2D */