removeDefaultAttributeValue: Special-case order attribute

Scour tried to handle "order" attribute as a SVGLength.  However, the
"order" attribute *can* consist of two integers according to the
[SVG 1.1 Specification] and SVGLength is not designed to handle that.

With this change, we now pretend that "order" is a string, which side
steps this issue.

[SVG 1.1 Specification]: https://www.w3.org/TR/SVG11/single-page.html#filters-feConvolveMatrixElementOrderAttribute

Closes: #189
Signed-off-by: Niels Thykier <niels@thykier.net>

scour/scour.py

@@ -1848,7 +1848,12 @@ default_attributes = [
     DefaultAttribute('offset', 0, elements=['feFuncA', 'feFuncB', 'feFuncG', 'feFuncR']),
     DefaultAttribute('operator', 'over', elements=['feComposite']),
     DefaultAttribute('operator', 'erode', elements=['feMorphology']),
-    DefaultAttribute('order', 3, elements=['feConvolveMatrix']),
+    # We pretend order is a string (because handling it as an
+    # SVGLength will cause issues when order is two integers).  Note
+    # that order must be exactly one or two integers (no units or
+    # fancy numbers), so working with it a string will generally just
+    # work.
+    DefaultAttribute('order', '3', elements=['feConvolveMatrix']),
     DefaultAttribute('pointsAtX', 0, elements=['feSpotLight']),
     DefaultAttribute('pointsAtY', 0, elements=['feSpotLight']),
     DefaultAttribute('pointsAtZ', 0, elements=['feSpotLight']),

testscour.py

@@ -1570,6 +1570,16 @@ class RemoveDefaultGradFYValue(unittest.TestCase):
                          'fy matching cy not removed')
 
 
+class RemoveDefaultAttributeOrderSVGLengthCrash(unittest.TestCase):
+
+    # Triggered a crash in v0.36
+    def runTest(self):
+        try:
+            scourXmlFile('unittests/remove-default-attr-order.svg')
+        except AttributeError:
+            self.fail("Processing the order attribute triggered an AttributeError ")
+
+
 class CDATAInXml(unittest.TestCase):
 
     def runTest(self):

unittests/remove-default-attr-order.svg

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink= "http://www.w3.org/1999/xlink">
+  <defs>
+    <filter id="filter" filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">
+      <feConvolveMatrix order="3 1" kernelMatrix="0.3333 0.3333 0.3333" edgeMode="none"/>
+    </filter>
+  </defs>
+  <!-- Use the filter (otherwise, scour discards it before it trips over it) -->
+  <image id="png" x="10" y="30" width="150" height="50" xlink:href="raster.png"
+         filter="url(#filter)"/>
+</svg>