decoder: Use glycin for decoding SVGs as well

Part-of: <https://gitlab.gnome.org/GNOME/loupe/-/merge_requests/267>

src/decoder/formats/mod.rs

@@ -54,7 +54,7 @@ impl std::fmt::Display for ImageFormat {
 
 #[derive(Clone, Debug, Default)]
 pub enum ImageDimensionDetails {
-    Svg((rsvg::Length, rsvg::Length)),
+    Svg(String),
     #[default]
     None,
 }