• Added something similar to futures::future::Abortable to abort Futures as soon as possible.
• Custom cursors are now await per window with a wasm_bindgen_futures::spawn_local() instead of going through a custom event loop event.
• Conditionally removed dependencies that aren't necessary in single-threaded Wasm (atomic-waker and concurrent-queue).
• Add CustomCursorBuilder::build_async(), to allow users to wait until a cursor has actually loaded and receive an appropriate error if loading wasn't successful.
• Add CustomCursor::from_animation() to create animated cursors from CustomCursors using the Web Animations API.

Addresses #3306.