Introduce explicit image::allocate API

core/src/renderer.rs

@@ -2,6 +2,7 @@
 #[cfg(debug_assertions)]
 mod null;
 
+use crate::image;
 use crate::{
     Background, Border, Color, Font, Pixels, Rectangle, Shadow, Size,
     Transformation, Vector,
@@ -62,6 +63,13 @@ pub trait Renderer {
 
     /// Resets the [`Renderer`] to start drawing in the `new_bounds` from scratch.
     fn reset(&mut self, new_bounds: Rectangle);
+
+    /// Creates an [`image::Allocation`] for the given [`image::Handle`] and calls the given callback with it.
+    fn allocate_image(
+        &mut self,
+        handle: &image::Handle,
+        callback: impl FnOnce(image::Allocation) + Send + 'static,
+    );
 }
 
 /// A polygon with four sides.