Comment out some of the requestIdleCallback to fix walkthrough crash

(closes #4533)

There are several issues here.. Here's a quick brain dump:

- the requestIdleCallbacks in map.js (scheduleRedraw) seem to be causing
crashiness in Firefox on fast zoom/unzoom, mousewheel, etc.. anything where
the view transform changes a lot and the redraws don't catch up.
(commented out, reverted back to lodash throttle)

- the requestIdleCallback worker queue in idle_worker.js seems to be causing
crashiness in Firefox and Chrome when exiting the walkthrough.  Something about
deferring the tile parsing as the user leaves the intro has a problem.
It might be an infinite `while` loop, not sure.
(commented out, reverted back to for loop)

- the requestIdleCallback in call_when_idle.js is only used to defer
tile loading in context.loadTiles() - this one seems fine, and actually
has maybe the biggest benefit for improving performance.
(left in)

modules/renderer/map.js

@@ -1,5 +1,6 @@
 import _compact from 'lodash-es/compact';
 import _map from 'lodash-es/map';
+import _throttle from 'lodash-es/throttle';
 import _values from 'lodash-es/values';
 
 import { set as d3_set } from 'd3-collection';
@@ -80,25 +81,27 @@ export function rendererMap(context) {
         .on('zoom', zoomPan);
 
     var _selection = d3_select(null);
-    var isRedrawScheduled = false;
-    var pendingRedrawCall;
 
-    function scheduleRedraw() {
-        // Only schedule the redraw if one has not already been set.
-        if (isRedrawScheduled) return;
-        isRedrawScheduled = true;
-        var that = this;
-        var args = arguments;
-        pendingRedrawCall = requestIdleCallback(function () {
-            // Reset the boolean so future redraws can be set.
-            isRedrawScheduled = false;
-            redraw.apply(that, args);
-        }, { timeout: 1400 });
-    }
+    var scheduleRedraw = _throttle(redraw, 750);
+    // var isRedrawScheduled = false;
+    // var pendingRedrawCall;
+    // function scheduleRedraw() {
+    //     // Only schedule the redraw if one has not already been set.
+    //     if (isRedrawScheduled) return;
+    //     isRedrawScheduled = true;
+    //     var that = this;
+    //     var args = arguments;
+    //     pendingRedrawCall = window.requestIdleCallback(function () {
+    //         // Reset the boolean so future redraws can be set.
+    //         isRedrawScheduled = false;
+    //         redraw.apply(that, args);
+    //     }, { timeout: 1400 });
+    // }
 
     function cancelPendingRedraw() {
-        isRedrawScheduled = false;
-        window.cancelIdleCallback(pendingRedrawCall);
+        scheduleRedraw.cancel();
+        // isRedrawScheduled = false;
+        // window.cancelIdleCallback(pendingRedrawCall);
     }
 
     function map(selection) {

modules/util/idle_worker.js

@@ -1,27 +1,34 @@
 export function utilIdleWorker(tasks, processor, callback) {
-    var processed = [];
-    var currentPos = 0;
-    var totalTasks = tasks.length;
-
-    function worker(deadline) {
-        while (deadline.timeRemaining() > 0 && currentPos < totalTasks) {
-            var result = processor(tasks[currentPos]);
-
-            // if falsy dont add to the processed list
-            if (result) processed.push(result);
-            currentPos++;
-        }
-
-        // more tasks are left, we might need more idleCallbacks
-        if (currentPos < totalTasks) {
-            return window.requestIdleCallback(function(deadline) {worker(deadline);});
-        }
-
-        // tasks are completed
-        return callback(processed);
+    var results = [], result;
+    for (var i = 0; i < tasks.length; i++) {
+        result = processor(tasks[i]);
+        if (result) results.push(result);
     }
+    callback(results);
 
-    window.requestIdleCallback(function(deadline) {worker(deadline);});
+    // var processed = [];
+    // var currentPos = 0;
+    // var totalTasks = tasks.length;
+
+    // function worker(deadline) {
+    //     while (deadline.timeRemaining() > 0 && currentPos < totalTasks) {
+    //         var result = processor(tasks[currentPos]);
+
+    //         // if falsy dont add to the processed list
+    //         if (result) processed.push(result);
+    //         currentPos++;
+    //     }
+
+    //     // more tasks are left, we might need more idleCallbacks
+    //     if (currentPos < totalTasks) {
+    //         return window.requestIdleCallback(function(deadline) {worker(deadline);});
+    //     }
+
+    //     // tasks are completed
+    //     return callback(processed);
+    // }
+
+    // window.requestIdleCallback(function(deadline) {worker(deadline);});
 }
 
 // shim