Don't compile mesa on mac, add cvwrap in case opencv isn't available,…

… tolerate lack of cython, and added normalization option to VertNormals.

cvwrap.py

@@ -0,0 +1,4 @@
+try:
+    import cv2
+except:
+    raise Exception('Failed to import cv2 from the OpenCV distribution. Please install OpenCV with Python support. OpenCV may either be installed from http://opencv.org or installed with package managers such as Homebrew (on Mac, http://brew.sh) or apt-get (on Ubuntu or Debian).')