title: R officer powerpoint complains with external image that is a pdf
body: <p>I want to add a graphic in the form of a pdf to a power point file created by officer. The following code works, in the sense that the powerpoint is created and the pdf is included. But to get to the powerpoint you have to let it repair itself. Here's an example</p>
  
  <pre><code>library(officer)
  library(magrittr)
  my_pres &lt;- read_pptx()
  testpdf &lt;- "test.pdf"
  extImgObs_pdf &lt;- external_img(src = testpdf)
  
  fp_3 &lt;- fp_text(italic = TRUE, color = "black", font.size = 14)
  my_pres &lt;- add_slide(x = my_pres, layout = 'Title Only', master = 'Office Theme')
  titleTextObserved &lt;- "some text for the title field"
  my_pres &lt;- ph_with( x = my_pres, value = fpar(ftext(titleTextObserved, fp_3)), location = ph_location_type(type = "title") )
  my_pres &lt;- ph_with( x = my_pres, value = extImgObs_pdf, location = ph_location(left = 0, top = 1.2) )
  
  print(my_pres, target = "presentations/cmip6/chillHours_Ensemble.pptx") %&gt;% browseURL()
  </code></pre>
  
  <p>With some arbitrary test.pdf file (I've generated it with the pdf driver in R and removed whitespace with the linux <code>pdfcrop</code> program), the above code generates a powerpoint. If you click the Repair ok message the powerpoint shows up as expected. But when you click Cancel rather than Repair, Powerpoint returns "Sorry, PowerPoint can't read ^O." Is there any way to remove the Repair message other than by manually clicking <code>Repair</code>?</p>