image/rendering: Use scale instead of scale-factor

`GdkSurface:scale` supports fractional scaling available as
"scale-monitor-framebuffer" on Mutter. Use this value everywhere
instead of the integer scale-factor

Closing #182

src/widgets/image/drag.rs

@@ -34,7 +34,7 @@ impl LpImage {
         let scale = f32::min(1., THUMBNAIL_SIZE / long_side as f32);
         let render_options = tiling::RenderOptions {
             scaling_filter: gsk::ScalingFilter::Trilinear,
-            scale_factor: self.scale_factor(),
+            scaling: self.scaling(),
             background_color: Some(self.background_color()),
         };