LaTeX: sync pdftex engine default imageresolution with pxunit

Closes: #8253

The 'pxunit' key from latex_elements instructs how to handle image
dimensions specified in px units.

But pdftex has \pdfimageresolution which is used when an image file does
not provide readable or legit values for the x and/or y resolution.

This commit syncs them: from 'pxunit' the default image resolution in
pixels per inch (an integer) is computed.

This way an image will behave the same if:

- it is loaded with no explicit size set, _and_ no readable image
  resolution data is readable from the file (or that data matches the
  'pxunit' setting)

- or a size is set in figure directive using px units and equal to the
  natural pixel size of the image,

This also with 'lualatex' but is ignored by with 'xelatex' and
'uplatex'.

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