Refactor some image traits a bit

- Use `Size<u32>` were applicable.
- Rename `TextureStore` to `image::Storage`.
- Rename `TextureStoreEntry` to `image::storage::Entry`.
- Wire up `viewport_dimensions` to `iced_glow` for `Svg`.

native/src/widget/image.rs

@@ -85,7 +85,7 @@ where
 {
     // The raw w/h of the underlying image
     let image_size = {
-        let (width, height) = renderer.dimensions(handle);
+        let Size { width, height } = renderer.dimensions(handle);
 
         Size::new(width as f32, height as f32)
     };
@@ -149,7 +149,7 @@ where
         _cursor_position: Point,
         _viewport: &Rectangle,
     ) {
-        let (width, height) = renderer.dimensions(&self.handle);
+        let Size { width, height } = renderer.dimensions(&self.handle);
         let image_size = Size::new(width as f32, height as f32);
 
         let bounds = layout.bounds();