scour: Make optimized default_attribute data structures

There are a lot of "DefaultAttribute"s and for a given tag, most of
the "DefaultAttribute"s are not applicable.  Therefore, we create two
data structures to assist us with only dealing with the attributes
that matter.

Here there are two cases:

 * Those that always matter.  These go into
   default_attributes_unrestricted list.
 * Those that matter only based on the node name.  These go into the
   default_attributes_restricted_by_tag with the node name as key
   (with the value being a list of matching attributes).

In the next commit, we will use those for optimizing the removal of
default attributes.

Signed-off-by: Niels Thykier <niels@thykier.net>

scour/scour.py

@@ -1876,6 +1876,20 @@ default_attributes = [
     DefaultAttribute('yChannelSelector', 'A', elements='feDisplacementMap')
 ]
 
+default_attributes_restricted_by_tag = defaultdict(list)
+default_attributes_unrestricted = []
+
+for attr in default_attributes:
+    if attr.elements is None:
+        # Applies to all tags
+        default_attributes_unrestricted.append(attr)
+        continue
+    if type(attr.elements) is str:
+        default_attributes_restricted_by_tag[attr.elements].append(attr)
+    else:
+        for tag in attr.elements:
+            default_attributes_restricted_by_tag[tag].append(attr)
+
 
 def taint(taintedSet, taintedAttribute):
     u"""Adds an attribute to a set of attributes.