Refactor texture image filtering

- Support only `Linear` or `Nearest`
- Simplify `Layer` groups
- Move `FilterMethod` to `Image` and `image::Viewer`

widget/src/image/viewer.rs

@@ -22,19 +22,21 @@ pub struct Viewer<Handle> {
     max_scale: f32,
     scale_step: f32,
     handle: Handle,
+    filter_method: image::FilterMethod,
 }
 
 impl<Handle> Viewer<Handle> {
     /// Creates a new [`Viewer`] with the given [`State`].
     pub fn new(handle: Handle) -> Self {
         Viewer {
+            handle,
             padding: 0.0,
             width: Length::Shrink,
             height: Length::Shrink,
             min_scale: 0.25,
             max_scale: 10.0,
             scale_step: 0.10,
-            handle,
+            filter_method: image::FilterMethod::default(),
         }
     }
 
@@ -329,6 +331,7 @@ where
                 image::Renderer::draw(
                     renderer,
                     self.handle.clone(),
+                    self.filter_method,
                     Rectangle {
                         x: bounds.x,
                         y: bounds.y,