add a test loading arcs with negative scale

This test currently fails, fix in the following commit.

The test loads an svg with a group with a transform with "scale(1,-1)".
This situation can mess up arc sweeps, resulting in corrupted paths.

test/negative-scale.svg

@@ -0,0 +1,19 @@
+<?xml version="1.0"?>
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
+<svg width="100mm" height="100mm" viewBox="-100 -200 500 500" xmlns="http://www.w3.org/2000/svg" version="1.1">
+    <g id="Sketch" transform="scale(1,-1)">
+        <path id="slot" d="
+            M 0 10
+            L 0 80
+            A 30 30 0 1 0 0 140
+            A 10 10 0 0 1 0 100
+            L 100 100
+            A 10 10 0 1 1 100 140
+            A 30 30 0 0 0 100 80
+            L 100 10
+            A 10 10 0 0 0 90 0
+            L 10 0
+            A 10 10 0 0 0 0 10
+        " stroke="#ff0000" stroke-width="0.35 px"/>
+    </g>
+</svg>

test/test_groups.py

@@ -5,11 +5,15 @@ $ python -m unittest test.test_groups.TestGroups.test_group_flatten
 """
 from __future__ import division, absolute_import, print_function
 import unittest
-from svgpathtools import Document, SVG_NAMESPACE, parse_path
+from svgpathtools import Document, SVG_NAMESPACE, parse_path, Line, Arc
 from os.path import join, dirname
 import numpy as np
 
 
+# When an assert fails, show the full error message, don't truncate it.
+unittest.util._MAX_LENGTH = 999999999
+
+
 def get_desired_path(name, paths):
     return next(p for p in paths
                 if p.element.get('{some://testuri}name') == name)
@@ -42,6 +46,22 @@ class TestGroups(unittest.TestCase):
         self.check_values(tf.dot(v_s), actual.start)
         self.check_values(tf.dot(v_e), actual.end)
 
+    def test_group_transform(self):
+        # The input svg has a group transform of "scale(1,-1)", which
+        # can mess with Arc sweeps.
+        doc = Document(join(dirname(__file__), 'negative-scale.svg'))
+        path = doc.paths()[0]
+        self.assertEqual(path[0], Line(start=-10j, end=-80j))
+        self.assertEqual(path[1], Arc(start=-80j, radius=(30+30j), rotation=0.0, large_arc=True, sweep=True, end=-140j))
+        self.assertEqual(path[2], Arc(start=-140j, radius=(20+20j), rotation=0.0, large_arc=False, sweep=False, end=-100j))
+        self.assertEqual(path[3], Line(start=-100j, end=(100-100j)))
+        self.assertEqual(path[4], Arc(start=(100-100j), radius=(20+20j), rotation=0.0, large_arc=True, sweep=False, end=(100-140j)))
+        self.assertEqual(path[5], Arc(start=(100-140j), radius=(30+30j), rotation=0.0, large_arc=False, sweep=True, end=(100-80j)))
+        self.assertEqual(path[6], Line(start=(100-80j), end=(100-10j)))
+        self.assertEqual(path[7], Arc(start=(100-10j), radius=(10+10j), rotation=0.0, large_arc=False, sweep=True, end=(90+0j)))
+        self.assertEqual(path[8], Line(start=(90+0j), end=(10+0j)))
+        self.assertEqual(path[9], Arc(start=(10+0j), radius=(10+10j), rotation=0.0, large_arc=False, sweep=True, end=-10j))
+
     def test_group_flatten(self):
         # Test the Document.paths() function against the
         # groups.svg test file.