Update AUCTeX to 11.88 (vanilla)
AUCTeX patch: emacsclient and high-res PNG icons
AUCTeX preview: high-res on Retina displays; improved UI
Always cache preview preamble.
(Grey out) non-functional preview menu entries.
Generate 2x size previews, then scale
them down again.

Provide `scale’ and `truedpi’ values for conversion: attribute for Emacs images rather than using a global variable to turn on “truedpi” resizing.

Currently using 0.8 factor for resizing. (Todo: make it work precisely for all display resolutions.)