use COMPUTE_THRESHOLD=0 on os x, update expected test images

tests/visual_tests/compare.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 
+import os
 import sys
 import mapnik
 
@@ -10,6 +11,11 @@ except ImportError:
 
 COMPUTE_THRESHOLD = 16
 
+# testcase images are generated on OS X
+# so they should exactly match
+if os.uname()[0] == 'Darwin':
+    COMPUTE_THRESHOLD = 0
+
 # returns true if pixels are not identical
 def compare_pixels(pixel1, pixel2, alpha=True):
     if pixel1 == pixel2: