image-loader: Add SVG support using librsvg

SVG[1] is a vector format which is quite widespread on the Linux
desktop, both for icons and for wallpapers.

librsvg[2] is a GNOME project doing rendering of SVG using cairo, which
Weston is already using.

This patch only adds support for rendering it at the internal dimensions
of the image, since all previous image formats supported are bitmap it
didn’t make sense to allow re-rendering at a different resolution.  A
future improvement would be to let the user specify the wanted size so
that the image will be rendered pixel perfect, without any scaling, even
on HiDPI outputs and such.

[1] https://en.wikipedia.org/wiki/Scalable_Vector_Graphics
[2] https://wiki.gnome.org/action/show/Projects/LibRsvg

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
2 jobs for librsvg-support in 9 minutes and 38 seconds (queued for 4 minutes and 16 seconds)
Status Job ID Name Coverage
  Build
passed #120385
build-native-autotools

00:04:02

passed #120386
build-native-meson

00:05:35