Drag and Drop: read incrementally

[rm-dr]

Is your feature request related to a problem? Please describe.

When dropping a large file into egui on the web, eframe always reads the whole file---even if file drop events aren't handled. Drag a particularly large file into egui, and eframe will try to load the whole thing into memory. This isn't ideal.

The code below is problematic:

egui/crates/eframe/src/web/events.rs

Lines 563 to 589 in ca36f3d

	if let Some(file) = files.get(i) {
	let name = file.name();
	let mime = file.type_();
	let last_modified = std::time::UNIX_EPOCH
	+ std::time::Duration::from_millis(file.last_modified() as u64);
	log::debug!("Loading {:?} ({} bytes)…", name, file.size());
	let future = wasm_bindgen_futures::JsFuture::from(file.array_buffer());
	let runner_ref = runner_ref.clone();
	let future = async move {
	match future.await {
	Ok(array_buffer) => {
	let bytes = js_sys::Uint8Array::new(&array_buffer).to_vec();
	log::debug!("Loaded {:?} ({} bytes).", name, bytes.len());
	if let Some(mut runner_lock) = runner_ref.try_lock() {
	runner_lock.input.raw.dropped_files.push(
	egui::DroppedFile {
	name,
	mime,
	last_modified: Some(last_modified),
	bytes: Some(bytes.into()),
	..Default::default()
	},
	);

Describe the solution you'd like

It would be nice to read dropped files in parts, and only on demand. Seems that Blob.stream() is how JS usually does it, but my knowledge of web is limited.

May be related to #3702