Refactor texture image filtering

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

graphics/src/primitive.rs

@@ -68,6 +68,8 @@ pub enum Primitive<T> {
     Image {
         /// The handle of the image
         handle: image::Handle,
+        /// The filter method of the image
+        filter_method: image::FilterMethod,
         /// The bounds of the image
         bounds: Rectangle,
     },