Port default DPI tracking to Rust

It's the only place from where we're accessing it already.
7 jobs for default-dpi in 25 minutes and 3 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #180369
fedora test

00:06:47

 
  Lint
manual #180371
allowed to fail manual
clippy
passed #180370
rustfmt

00:00:24

passed #180372
tarball

00:10:18

 
  Cross Distro
failed #180375
debian_i386:testing

00:03:32

passed #180374
debian:testing

00:07:55

passed #180373
fedora:rawhide

00:07:49

 
Name Stage Failure
failed
debian_i386:testing Cross Distro
make[2]: Leaving directory '/builds/cmartin/librsvg/_build'
make[1]: *** [Makefile:1461: all-recursive] Error 1
make[1]: Leaving directory '/builds/cmartin/librsvg/_build'
make: *** [Makefile:964: all] Error 2
Uploading artifacts...
WARNING: _build/tests/*.log: no matching files
WARNING: _build/tests/output/: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1