Move image/svg handling into iced_graphics

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.

wgpu/src/backend.rs

@@ -99,7 +99,7 @@ impl Backend {
         }
 
         #[cfg(any(feature = "image_rs", feature = "svg"))]
-        self.image_pipeline.trim_cache();
+        self.image_pipeline.trim_cache(device, encoder);
     }
 
     fn flush(