Don't fail if pngcrush is not found

If pngcrush is not installed, don't delete the packed file and
output it without compressing with pngcrush.

anim_encoder.py

@@ -196,8 +196,12 @@ def generate_animation(anim_name):
     packed = packed[0:allocator.num_used_rows]
 
     misc.imsave(anim_name + "_packed_tmp.png", packed)
-    os.system("pngcrush -q " + anim_name + "_packed_tmp.png " + anim_name + "_packed.png")
-    os.system("rm " + anim_name + "_packed_tmp.png")
+    # Don't completely fail if we don't have pngcrush
+    if os.system("pngcrush -q " + anim_name + "_packed_tmp.png " + anim_name + "_packed.png") == 0:
+        os.system("rm " + anim_name + "_packed_tmp.png")
+    else:
+        print "pngcrush not found, output will not be larger"
+        os.system("mv " + anim_name + "_packed_tmp.png " + anim_name + "_packed.png")
 
     # Generate JSON to represent the data
     times = [t for t, f in frames]