Doc pdfs: Use PNG for actdiag output, avoid crash

When actdiag uses PDF output, it issues a warning, which crashes when using Sphinx 2, because builder.warn() is no longer available. The warning is about not finding a font, so an alternative is to set actdiag_fontpath to a true-type font, but we don't have one readily available that we are already using. VTK has some in testing data... PNG seemed easiest, and looks fine.

@dcthomp Tested PDF and epub generation locally - hopefully last fix for master to completely work on RTD...