bug#37689: Fringe pixmaps, widgets, etc. look ridiculously tiny in hidpi screen

[Carlos Pita]

I decided to follow a more incremental strategy because I feel this is
getting unwieldy.

I split the issue in three parts for now:

1. Expose scale factor in the rif (patch in
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=37770)
2. Implement image scaling in Cairo and maybe X11 backends (this ticket)
3. Some code cleanups / refactors pertaining the initialization
sequence of backends (patch in
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=37755)

1 is a must-have for 2, 3 is a nice-to-have.

I'm writing a proper patch for 2 on top of my patch for 1 that I will
be attaching here soon.

Are you ok with this strategy? It's not the one-hack-to-rule-them-all
approach envisioned at the beginning but it simplify things a lot for
anyone wanting to make macos and win backends actually scale their
bitmaps, which shouldn't be difficult at all (or even necessary, I
still have doubts regarding macos). Plus it hopefully brings some code
improvements and a necessary API.