snapdiff: Compare .bmp images too.

Quite common too.
diff --git a/scripts/snapdiff.py b/scripts/snapdiff.py
index b7bad72..ddc2bf7 100755
--- a/scripts/snapdiff.py
+++ b/scripts/snapdiff.py
@@ -121,10 +121,11 @@
 
 
 def is_image(path):
-    return \
-        path.endswith('.png') \
-        and not path.endswith('.diff.png') \
-        and not path.endswith('.thumb.png')
+    name = os.path.basename(path)
+    name, ext1 = os.path.splitext(name)
+    name, ext2 = os.path.splitext(name)
+    print name, ext1, ext2
+    return ext1 in ('.png', '.bmp') and ext2 not in ('.diff', '.thumb')
 
 
 def find_images(prefix):