Allow to load an image from memory

New `image::Handle` opaque type uniquely identifying some `image::Data`,
allowing reliable caching.

wgpu/src/renderer.rs

@@ -229,9 +229,9 @@ impl Renderer {
                     border_radius: *border_radius as f32,
                 });
             }
-            Primitive::Image { path, bounds } => {
+            Primitive::Image { handle, bounds } => {
                 layer.images.push(Image {
-                    path: path.clone(),
+                    handle: handle.clone(),
                     position: [bounds.x, bounds.y],
                     scale: [bounds.width, bounds.height],
                 });