Allow to load an image from memory

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

wgpu/src/renderer/widget/image.rs

@@ -2,14 +2,18 @@ use crate::{Primitive, Renderer};
 use iced_native::{image, Layout, MouseCursor};
 
 impl image::Renderer for Renderer {
-    fn dimensions(&self, path: &str) -> (u32, u32) {
-        self.image_pipeline.dimensions(path)
+    fn dimensions(&self, handle: &image::Handle) -> (u32, u32) {
+        self.image_pipeline.dimensions(handle)
     }
 
-    fn draw(&mut self, path: &str, layout: Layout<'_>) -> Self::Output {
+    fn draw(
+        &mut self,
+        handle: image::Handle,
+        layout: Layout<'_>,
+    ) -> Self::Output {
         (
             Primitive::Image {
-                path: String::from(path),
+                handle,
                 bounds: layout.bounds(),
             },
             MouseCursor::OutOfBounds,