Add very basic SVG support

We need to refactor, so that SVGs are pre-rendered on each change
of scaling by librsvg directly, because some elements may be rasterized.

It would be best to also support building against resvg.

README.adoc

@@ -2,7 +2,7 @@ fastiv
 ======
 
 'fastiv' is a fast image viewer, supporting BMP, PNG, GIF, JPEG, and optionally
-RAW pictures.  Currently, it's not particularly usable.
+RAW and SVG pictures.  Currently, it's not particularly usable.
 
 Non-goals
 ---------
@@ -19,7 +19,7 @@ Building and Running
 --------------------
 Build dependencies: Meson, pkg-config +
 Runtime dependencies: gtk+-3.0, pixman-1, shared-mime-info, libpng>=1.5.4,
-libturbojpeg, LibRaw (optional)
+libturbojpeg, LibRaw (optional), librsvg-2.0 (optional)
 
  $ git clone --recursive https://git.janouch.name/p/fastiv.git
  $ meson builddir