Improve inheritance diagrams

When Graphviz is given a page size (which Sphinx does by default), it
will layout the whole graph, and then scale the _entire_ thing down to
fit. This means that text and lines on large diagrams are inconsistent
with ones that fit.

Additionally, the default size is larger than the content width which
means the browser will also scale the image down a bit. But the URL map
is _not_ scaled down, so the links will appear in the wrong spot.

To fix this, set the page size in Graphviz very large, and make the
diagram scrollable with CSS.

Plus a few minor tweaks to font and line sizes now that images are a
consistent scale.

doc/_static/mpl.css

@@ -99,3 +99,13 @@ table.property-table td {
     display: inline-block;
     margin: 0 0.5em;
 }
+
+/* Make inheritance images have a scroll bar if necessary. */
+div.graphviz {
+    border: 1px solid lightgrey;
+    max-height: 50em;
+    overflow: auto;
+}
+img.graphviz.inheritance {
+    max-width: none;
+}

doc/conf.py

@@ -619,7 +619,12 @@ def js_tag_with_cache_busting(js):
 
 numpydoc_show_class_members = False
 
-inheritance_node_attrs = dict(fontsize=16)
+# We want to prevent any size limit, as we'll add scroll bars with CSS.
+inheritance_graph_attrs = dict(dpi=100, size='1000.0', splines='polyline')
+# Also remove minimum node dimensions, and increase line size a bit.
+inheritance_node_attrs = dict(height=0.02, margin=0.055, penwidth=1,
+                              width=0.01)
+inheritance_edge_attrs = dict(penwidth=1)
 
 graphviz_dot = shutil.which('dot')
 # Still use PNG until SVG linking is fixed