Among docx, pptx, rtf, and odt, pptx is the only format that does not support width and height.

Do we need warning when output is pptx and out.width and out.height are specified?

Originally posted by @atusy in #1746 (comment)

Following #1746 we never added the warning in knitr. Pandoc still does not support width and height for PPTX output (jgm/pandoc#4586)

It would be useful to add a warning about this when output format is detected to be pptx.

cc @apreshill