- 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`.
The `TextureStore` trait is implemented by the atlas, and can also be
implemented in the glow renderer or in a software renderer.
The API here may be improved in the future, but API stability is
presumably not a huge issue since these types will only be used by
renderer backends.
