pdf export paper_bgcolor turns black when using image magick to convert to jpg

[cldougl]

Now after exporting a pdf image and using image magick to convert to a jpg the paper_bgcolor becomes black:

[rafael-andrade]

I was having the same issue using another library. I was converting it to JPEG image, the default color is black. So I changed it to PNG before export to PDF.

[etpinard]

Moved to plotly/orca#81