[NTG-context] Disable reuse of figures

[Aditya Mahajan]

Hi,

This is related to a bug report for the filter module:
https://github.com/adityam/filter/issues/60

Normally, when the same figure is included twice, context only includes the figure once and refers to the included figure the second time. The option `object=no` to externalfigure is supposed to prevent that, but it doesn't work. For example, consider the following example the bug-report uses graphviz as the external program, but I have changed that to context to keep things self contained)

\usemodule[filter]

\defineexternalfilter
  [graphviz]
  [
    filter=context --purge,
    output=\externalfilterbasefile.pdf,
    readcommand=\ReadPDF,
  ]

\def\ReadPDF#1%
    {\externalfigure[#1][object=no]}

\starttext

  Circle:
  \startgraphviz
    \startMPpage
      draw fullcircle scaled 1cm;
    \stopMPpage
  \stopgraphviz

  Square:
  \startgraphviz
    \startMPpage
      draw fullsquare scaled 1cm;
    \stopMPpage
  \stopgraphviz

\stoptext

Behind the scenes, the first \startgraphviz ... \stopgraphviz environment writes the content to an external file (\jobname-temp-graphviz.tmp), runs `context --purge \jobname-temp-graphviz.tmp`, which creates `\jobname-temp-graphviz.pdf`. The `\ReadPDF` macro then includees the file (with object=no).

At the next \startgraphviz .. \stopgraphviz, the same process is repeated. The `\jobname-temp-graphviz.pdf` file is overwritten and the new file should have been included. But it is not. The output contains the first image twice. 

I thought that object=no should have prevented the reuse. In fact, if I compile the file with luatex, I get an error message:

! error:  (pdf inclusion): file has changed '12-two-outputs-temp-graphviz.pdf'
mtx-context     | fatal error: return code: 256

Any idea why object=no is not working?

Thanks,
Aditya