LaTeX: sync pdftex engine default imageresolution with pxunit

Closes: sphinx-doc#8253

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

pdftex has another setting \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: this way if an image will behave the same if
loaded with no explicit size set, or with one such size is set using px
unit, _and_ the graphics file does not have readable image resolution
data (or that data matches the 'pxunit' setting).

Works also with 'lualatex' as latex_engine.

Does not work with 'xelatex' and 'uplatex'.

CHANGES

@@ -68,6 +68,8 @@ Bugs fixed
   ``:unknown:doc:``)
 * #8711: LaTeX: backticks in code-blocks trigger latexpdf build warning (and font
   change) with late TeXLive 2019
+* #8253: LaTeX: Figures with no size defined get overscaled (compared to images
+  with size explicitly set in pixels) (fixed for `'pdflatex'/'lualatex'` only)
 
 Testing
 --------

sphinx/templates/latex/latex.tex_t

@@ -13,6 +13,9 @@
 \ifdefined\pdfpxdimen
    \let\sphinxpxdimen\pdfpxdimen\else\newdimen\sphinxpxdimen
 \fi \sphinxpxdimen=<%= pxunit %>\relax
+\ifdefined\pdfimageresolution
+    \pdfimageresolution= \numexpr \dimexpr1in\relax/\sphinxpxdimen\relax
+\fi
 <% if use_xindy -%>
 %% turn off hyperref patch of \index as sphinx.xdy xindy module takes care of
 %% suitable \hyperpage mark-up, working around hyperref-xindy incompatibility