svg: store image resources in the svg struct

This makes it consistent with xml resources. It also allows images
to be cached in case they are rendered more than once.
The images are loaded lazily during rendering which goes in the
opposite direction with the goal of having an immutable Svg
object, but at least this is the same of other resources. Beside
images are just used in two places, and filters/images was
already doing a hack to load the image at rendering time.
7 jobs for image in 26 minutes and 59 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #188349
fedora test

00:06:53

 
  Lint
manual #188351
allowed to fail manual
clippy
passed #188350
rustfmt

00:00:40

passed #188352
tarball

00:10:37

 
  Cross Distro
passed #188355
debian_i386:testing

00:09:27

passed #188354
debian:testing

00:08:44

passed #188353
fedora:rawhide

00:08:31