image: Use mipmaps for zoom-out and NN for zoom-in

Before we always used "linear" filtering. This gave
a harsh image with moire patterns when zooming out
and washed out images when zooming in.

Closes #68, #71

Part-of: <https://gitlab.gnome.org/Incubator/loupe/-/merge_requests/125>

src/decoder/formats/mod.rs

@@ -27,6 +27,10 @@ impl ImageFormat {
             ImageFormat::AnimatedGif | ImageFormat::AnimatedWebP | ImageFormat::AnimatedPng
         )
     }
+
+    pub fn is_svg(&self) -> bool {
+        matches!(self, Self::Svg)
+    }
 }
 
 impl std::fmt::Display for ImageFormat {