Introduce explicit image::allocate API

runtime/src/image.rs

@@ -0,0 +1,24 @@
+//! Allocate images explicitly to control presentation.
+use crate::core::image::Handle;
+use crate::futures::futures::channel::oneshot;
+use crate::task::{self, Task};
+
+pub use crate::core::image::Allocation;
+
+/// An image action.
+#[derive(Debug)]
+pub enum Action {
+    /// Allocates the given [`Handle`].
+    Allocate(Handle, oneshot::Sender<Allocation>),
+}
+
+/// Allocates an image [`Handle`].
+///
+/// When you obtain an [`Allocation`] explicitly, you get the guarantee
+/// that using a [`Handle`] will draw the corresponding image immediately
+/// in the next frame.
+pub fn allocate(handle: Handle) -> Task<Allocation> {
+    task::oneshot(|sender| {
+        crate::Action::Image(Action::Allocate(handle, sender))
+    })
+}