Refactor texture image filtering

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

wgpu/src/layer/image.rs

@@ -10,6 +10,9 @@ pub enum Image {
         /// The handle of a raster image.
         handle: image::Handle,
 
+        /// The filter method of a raster image.
+        filter_method: image::FilterMethod,
+
         /// The bounds of the image.
         bounds: Rectangle,
     },