Fix reduce_4() to make it consistent with reduce_8(). This was breaking tiles which were compressing down to a 16 entry colormap. All the pixels were being set to the alpha index.

include/mapnik/png_io.hpp

@@ -126,7 +126,7 @@ namespace mapnik {
             mapnik::rgb c((val)&0xff, (val>>8)&0xff, (val>>16) & 0xff);
             byte index = tree.quantize(c);
             if (x%2 >  0) index = index<<4;
-            if ((val>>24)^0x80) index = 0;//alfa
+            if (!((val>>24)&0x80)) index = 0;//alfa
             row_out[x>>1] |= index;
          }
       }