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.

fastiv.desktop

@@ -8,4 +8,4 @@ Terminal=false
 StartupNotify=true
 Categories=Graphics;2DGraphics;Viewer;
 # TODO(p): Generate this list from source files.
-MimeType=image/png;image/bmp;image/gif;image/jpeg;image/x-dcraw;
+MimeType=image/png;image/bmp;image/gif;image/jpeg;image/x-dcraw;image/svg+xml;