Fix matplotlib#4230: Don't overflow buffer with sketch path.

There's no good way to estimate the length of the string when
sketch is turned on, so I've just bitten the bullet and made
the string dynamically reallocatable after all.  It's a bit of
a performance regression in that there is always a string copy
upon creating the Python string object, but at least now this
should work for arbitrary path transformations.

Author: mdboom

lib/matplotlib/backends/backend_svg.py

@@ -540,21 +540,23 @@ def option_image_nocomposite(self):
         else:
             return not rcParams['image.composite_image']
 
-    def _convert_path(self, path, transform=None, clip=None, simplify=None):
+    def _convert_path(self, path, transform=None, clip=None, simplify=None,
+                      sketch=None):
         if clip:
             clip = (0.0, 0.0, self.width, self.height)
         else:
             clip = None
         return _path.convert_to_string(
-            path, transform, clip, simplify, None, 6,
+            path, transform, clip, simplify, sketch, 6,
             [b'M', b'L', b'Q', b'C', b'z'], False).decode('ascii')
 
     def draw_path(self, gc, path, transform, rgbFace=None):
         trans_and_flip = self._make_flip_transform(transform)
         clip = (rgbFace is None and gc.get_hatch_path() is None)
         simplify = path.should_simplify and clip
         path_data = self._convert_path(
-            path, trans_and_flip, clip=clip, simplify=simplify)
+            path, trans_and_flip, clip=clip, simplify=simplify,
+            sketch=gc.get_sketch_params())
 
         attrib = {}
         attrib['style'] = self._get_style(gc, rgbFace)