Make retracediff.py get the images from glretrace via stdout.

Much better performance, as no need to compress and write images to disk,
and also more synchronicity between child processes.
diff --git a/scripts/snapdiff.py b/scripts/snapdiff.py
index 60d9ae9..0ed3937 100755
--- a/scripts/snapdiff.py
+++ b/scripts/snapdiff.py
@@ -48,8 +48,15 @@
     '''Image comparer.'''
 
     def __init__(self, ref_image, src_image, alpha = False):
-        self.ref_im = Image.open(ref_image)
-        self.src_im = Image.open(src_image)
+        if isinstance(ref_image, basestring):
+            self.ref_im = Image.open(ref_image)
+        else:
+            self.ref_im = ref_image
+
+        if isinstance(src_image, basestring):
+            self.src_im = Image.open(src_image)
+        else:
+            self.src_im = src_image
 
         # Ignore
         if not alpha: