single page pdf_convert() should not format filenames

[ThierryO]

Maybe only format filenames when length(filenames) < 2 && length(pages) > 1

pdf_convert("cover.pdf", format = "png", pages = 1, filenames = "cover.png")
Converting page 1 to cover.png... done!
[1] "cover.png"
Warning message:
In sprintf(filenames, pages, format) :
  2 arguments not used by format 'cover.png'

pdftools/R/render.R

Line 76 in 92076d2

if(length(filenames) < 2){