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`.

wgpu/src/image/atlas/allocator.rs

@@ -46,10 +46,10 @@ impl Region {
         (rectangle.min.x as u32, rectangle.min.y as u32)
     }
 
-    pub fn size(&self) -> (u32, u32) {
+    pub fn size(&self) -> iced_graphics::Size<u32> {
         let size = self.allocation.rectangle.size();
 
-        (size.width as u32, size.height as u32)
+        iced_graphics::Size::new(size.width as u32, size.height as u32)
     }
 }