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/allocation.rs

@@ -1,5 +1,7 @@
 use crate::image::atlas::{self, allocator};
 
+use iced_graphics::Size;
+
 #[derive(Debug)]
 pub enum Allocation {
     Partial {
@@ -19,10 +21,10 @@ impl Allocation {
         }
     }
 
-    pub fn size(&self) -> (u32, u32) {
+    pub fn size(&self) -> Size<u32> {
         match self {
             Allocation::Partial { region, .. } => region.size(),
-            Allocation::Full { .. } => (atlas::SIZE, atlas::SIZE),
+            Allocation::Full { .. } => Size::new(atlas::SIZE, atlas::SIZE),
         }
     }