limit the maximum number of requests made per second

This should avoid crashing or getting banned from the server from which the tiles are downloaded
(which can be the same as the server from which dezoomify is served, when using google arts for instance)

dezoomers/arts-culture.js

@@ -48,7 +48,7 @@ async function getTileURL(
 	const tile_path = await compute_signed_path(path, token, x, y, z);
 	const tile_url = ZoomManager.resolveRelative("/"+tile_path, origin);
 	const buffer = await new Promise(accept =>
-		ZoomManager.getFile(tile_url, { type: 'binary' }, accept)
+		ZoomManager.getFile(tile_url, { type: 'binary', is_tile: true }, accept)
 	);
 	const tile = await decrypt_image({ buffer });
 	const blob = new Blob([tile], { type: "image/jpeg" });

zoommanager.js

@@ -132,6 +132,10 @@ Update the state of the progress bar.
 @param {String} description current state description
 */
 UI.updateProgress = function (percent, text) {
+	if (!percent) {
+		document.getElementById("percent").innerHTML = text;
+		return;
+	}
 	percent = parseInt(percent);
 	document.getElementById("percent").innerHTML = text + ' (' + percent + "%)";
 	document.getElementById("progressbar").style.width = percent + "%";
@@ -311,13 +315,15 @@ ZoomManager.defaultRender = function (data) {
 	nextTile();
 };
 
+ZoomManager.MAX_REQUESTS_PER_SECOND = 5;
+
 /**
 @function nextTick
 Call a function, but not immediatly
 @param {Function} f - the function to call
 */
 ZoomManager.nextTick = function (f) {
-	return setTimeout(f, 3);
+	return setTimeout(f, 1000 / ZoomManager.MAX_REQUESTS_PER_SECOND);
 };
 
 /**
@@ -388,7 +394,7 @@ ZoomManager.open = function (url) {
 /**
 Call callback with the contents of the page at url
 @param {string} url
-@param {{type:String, allow_failure?: boolean, error_callback: (err:string)=>any}} params
+@param {{type:String, allow_failure?: boolean, error_callback: (err:string)=>any, is_tile?: boolean}} params
 @param {fileCallback} callback - callback to call when the file is loaded
 */
 ZoomManager.getFile = function (url, params, callback) {
@@ -415,7 +421,8 @@ ZoomManager.getFile = function (url, params, callback) {
 	xhr.open("GET", requesturl, true);
 
 	xhr.onloadstart = function () {
-		ZoomManager.updateProgress(1, "Sent a request in order to get information about the image...");
+		if (!params.is_tile)
+			ZoomManager.updateProgress(0, "Sent a request in order to get information about the image...");
 	};
 	xhr.onerror = function (e) {
 		onerror("Unable to connect to the proxy server " +