Fix billboard and label clamping

Turns out that billboard and label clamping were fundamentally broken
because the `QuadtreePrimitive` was processing the tile queue in the wrong
order.  It was always pulling new tiles from the back of the array rather
than the front, which meant that data would get processed in the wrong
order causing old tiles to take precedence over newer tiles.

Addtiionally, there was a bad if block in `Label.js` which caused the
initial position of the individual label billboards to not be properly set
when clamping was on, instead we should always set the positions before
calling `_updateClamping` (if needed).

Fixes #4396 and #4062

Source/Scene/Label.js

@@ -166,15 +166,15 @@ define([
                 if (!Cartesian3.equals(position, value)) {
                     Cartesian3.clone(value, position);
 
-                    if (this._heightReference === HeightReference.NONE) {
-                        var glyphs = this._glyphs;
-                        for (var i = 0, len = glyphs.length; i < len; i++) {
-                            var billboard = glyphs[i].billboard;
-                            if (defined(billboard)) {
-                                billboard.position = value;
-                            }
+                    var glyphs = this._glyphs;
+                    for (var i = 0, len = glyphs.length; i < len; i++) {
+                        var billboard = glyphs[i].billboard;
+                        if (defined(billboard)) {
+                            billboard.position = value;
                         }
-                    } else {
+                    }
+
+                    if (this._heightReference !== HeightReference.NONE) {
                         this._updateClamping();
                     }
                 }

Source/Scene/QuadtreePrimitive.js

@@ -617,11 +617,7 @@ define([
         var ellipsoid = projection.ellipsoid;
 
         while (tilesToUpdateHeights.length > 0) {
-            var tile = tilesToUpdateHeights[tilesToUpdateHeights.length - 1];
-            if (tile !== primitive._lastTileUpdated) {
-                primitive._lastTileIndex = 0;
-            }
-
+            var tile = tilesToUpdateHeights[0];
             var customData = tile.customData;
             var customDataLength = customData.length;
 
@@ -682,11 +678,11 @@ define([
             }
 
             if (timeSliceMax) {
-                primitive._lastTileUpdated = tile;
                 primitive._lastTileIndex = i;
                 break;
             } else {
-                tilesToUpdateHeights.pop();
+                primitive._lastTileIndex = 0;
+                tilesToUpdateHeights.shift();
             }
         }
     }