image: fix export of paletted PNGs to jpeg
image: fix export of paletted PNGs to jpeg

 * Fix rendering of some PNGs that could not be converted to jpeg.
   Paletted pngs can't be directly converted to jpeg, they need to be
   converted to RGB mode first.  The conversion can be ugly, so a
   better approach will need to be found, but this will do for now.

felliott/modular-file-renderer,CenterForOpenScience/modular-file-renderer,CenterForOpenScience/modular-file-renderer,CenterForOpenScience/modular-file-renderer,felliott/modular-file-renderer,felliott/modular-file-renderer,CenterForOpenScience/modular-file-renderer,felliott/modular-file-renderer