graphviz: same map id is assigned for different graphs

[tk0miya]

Problem

• When the document has multiple graphviz graphs
• same map id is assigned for them

Procedure to reproduce the problem

.. graphviz::

   digraph {
     A [href="URL"];
   }

.. graphviz::

   digraph {
     B [href="URL"];
   }

Error logs / results

Following HTML is generated. There are multiple <map> tags having id="#%3". usemap attribute is also.

<img src="_images/graphviz-1d4985d223da0b0b87d99a9a48f61a022e1283be.png" alt="digraph {
  A [href=&quot;URL&quot;];
}" usemap="#%3" />
<map id="%3" name="%3">
<area shape="poly" id="node1" href="URL" title="A" alt="" coords="77,29,76,22,70,15,62,10,52,7,41,5,30,7,20,10,12,15,7,22,5,29,7,37,12
,43,20,49,30,52,41,53,52,52,62,49,70,43,76,37"/>
</map>
<img src="_images/graphviz-573176d0e453cc933d1043e425714fda81b96309.png" alt="digraph {
  B [href=&quot;URL&quot;];
}" usemap="#%3" />
<map id="%3" name="%3">
<area shape="poly" id="node1" href="URL" title="B" alt="" coords="77,29,76,22,70,15,62,10,52,7,41,5,30,7,20,10,12,15,7,22,5,29,7,37,12,43,20,49,30,52,41,53,52,52,62,49,70,43,76,37"/>
</map>

Expected results

• Generate valid HTML and correct clickable maps

Environment info

• OS: Mac
• Python version: 2.7.14
• Sphinx version: 1.6.6

[tk0miya]

Note:
%3 is comes from graphviz (see https://gitlab.com/graphviz/graphviz/issues/1327).
Anyway Sphinx should assign better IDs to every graphs.

[tk0miya]

Fixed by #4515