build_pptx and build_thumbnail build to 4:3 ratio always

[mattansb]

These function seem to ignore the actual aspect ratio of the xaringan (html) slides.

[jhelvy]

Yup, this is a problem. I'll see if I can fix this.

[jhelvy]

This should now be fixed for build_png() (I depreciated build_thumbnail()).

The aspect ratio is still off for pptx output types though.

[jhelvy]

This is now fixed with a somewhat hacky approach, but it appears to work. For now it only supports the default 4:3 ratio and 16:9.

[mattansb]

Thanks!