Debounce autosave & multi-user session sync

- When holding down Ctrl+Z or Ctrl+Shift+Z, it will now save only once or twice (at the end and usually the begining), instead of saving/syncing constantly, causing it to lag and not even show what's going on
- This fixes a situation where redo-after-cancel wouldn't work:

User cancels operation
	(in cancel():) op finalized, creating undoable -> sync to DB
	(in cancel():) cancel goes back to history node at start of cancel -> sync to DB
DB syncs first update -> undoable("Session Sync")
DB syncs second update -> undoable("Session Sync")
The history ends on "Session Sync", on a branch separate from the canceled operation.

src/helpers.js

@@ -33,6 +33,36 @@ function E(t){
 	return document.createElement(t);
 }
 
+/** Returns a function, that, as long as it continues to be invoked, will not
+be triggered. The function will be called after it stops being called for
+N milliseconds. If `immediate` is passed, trigger the function on the
+leading edge, instead of the trailing. */
+function debounce(func, wait_ms, immediate) {
+	let timeout;
+
+	return function() {
+		const context = this;
+		const args = arguments;
+
+		const later = ()=> {
+			timeout = null;
+			if (!immediate) {
+				func.apply(context, args);
+			}
+		};
+
+		const callNow = immediate && !timeout;
+
+		clearTimeout(timeout);
+
+		timeout = setTimeout(later, wait_ms);
+
+		if (callNow) {
+			func.apply(context, args);
+		}
+	};
+}
+  
 function memoize_synchronous_function(func) {
 	const cache = {};
 	return (...args)=> {

src/sessions.js

@@ -100,7 +100,7 @@
 			const lsid = `image#${session_id}`;
 			log(`local storage id: ${lsid}`);
 			// save image to storage
-			const save_image_to_storage = () => {
+			const save_image_to_storage = debounce(() => {
 				const save_paused = handle_data_loss();
 				if (save_paused) {
 					return;
@@ -117,7 +117,7 @@
 						}
 					}
 				});
-			};
+			}, 100);
 			storage.get(lsid, (err, uri) => {
 				if (err) {
 					if (localStorageAvailable) {
@@ -331,7 +331,7 @@
 			});
 			let previous_uri;
 			// let pointer_operations = []; // the multiplayer syncing stuff is a can of worms, so this is disabled
-			const write_canvas_to_database = () => {
+			const write_canvas_to_database = debounce(() => {
 				const save_paused = handle_data_loss();
 				if (save_paused) {
 					return;
@@ -348,7 +348,7 @@
 				else {
 					log("(don't write canvas data to Firebase; it hasn't changed)");
 				}
-			};
+			}, 100);
 			let ignore_session_update = false;
 			$G.on("session-update.session-hook", ()=> {
 				if (ignore_session_update) {