+ The symptom is that 'dvir' does not work on Cairo devices in Ubuntu 20.04, 
  not even ...

  library(dvir)
  grid.latex("$x - \\mu$")

  ... which just produces three missing character glyphs.

+ The problem is that Ubuntu 20.04 has Pango > 1.44, which means that 
  Cairo devices refuse to use Type 1 fonts.

  (see https://developer.r-project.org/Blog/public/2020/04/17/changes-to-symbol-fonts-for-cairo-graphics-devices/index.html)

  (this was discovered on Ubuntu 20.04, but it will occur on any OS
   with recent Pango versions)

  TeX (latex) uses Type 1 versions of the Computer Modern fonts by default,
  so 'dvir' correctly finds a Type 1 font (like cmmi12.pfb) and 
  attempts to draw text with that font and that produces garbage
  (on a Cairo device).

+ The solution is two-fold (at least):

  1. 
  We need to get TeX to use a TrueType or OpenType font (NOT a Type 1 font).

  This turns out to be pretty easy.  

  FIRST, we use LuaTeX (lualatex) instead of TeX (latex), because that
  makes it easy to select system fonts.

  TEHN, we can use something like ...

  \usepackage{fontspec} 
  \setmainfont[Mapping=text-tex]{Lato Light} 

  ... to set the main text font and something like ...

  \usepackage{unicode-math}
  % \setmathfont{latinmodern-math}

  ... to set the math font.  

  This means that lualatex will make use of a TrueType or OpenType font,
  so 'dvir' will find a TrueType or OpenType font (like latinmodern-math.otf)
  and it will draw text correctly (on a Cairo device).  
  
  2. 
  We need to fix a couple of things in 'dvir'.

  a)
  Add support for OpenType fonts to 'dvir'

  To do this we make use of the systemfonts::system_fonts() table
  (in addition to extrafont::fonttable(), because that only contains 
   TrueType fonts)
  to look up font information based on the font file that we get from
  the LuaTeX DVI output (plus luaotfload-tool).
  That does not get us quite as much font info (it does not contain .afm
  file info), but that is ok for drawing on Cairo devices (because we
  get the metric info from the font file via ttx).
  This could be a problem in the future if we try to add support to 'dvir'
  for LuaTeX with non-ComputerModern fonts on non-Cairo devices (like pdf()).

  b)
  Improve support for set3 operations in DVI output from LuaTeX

  This was only set up to handle a non-UNICODE character (where the first
  byte is 0x0f), but it can also just be a 3-byte UNICODE character
  (in UTF32BE format).

+ NOTE that the solution requires BOTH the new version of 'dvir'
  AND the user to make use of fontspec and/or unicode-math in their
  TeX code AND the user to use the LuaTeX engine.