decoder: Use glycin for image loading expect SVG

This gives us sandboxed image decoding and others the
option to extend Loupe with other image formats.

src/decoder/formats/svg.rs

@@ -15,7 +15,7 @@
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
 
-///! Decode using librsvg
+//! Decode using librsvg
 use super::*;
 use crate::decoder::tiling::{self, FrameBufferExt};
 use crate::decoder::TileRequest;