new DEMData and getImageData measurement patch

dem-measure.patch

diff --git a/src/source/raster_dem_tile_source.js b/src/source/raster_dem_tile_source.js
index 1937a103d..e7c6cafc2 100644
--- a/src/source/raster_dem_tile_source.js
+++ b/src/source/raster_dem_tile_source.js
@@ -55,7 +55,10 @@ class RasterDEMTileSource extends RasterTileSource implements Source {
                 delete (img: any).cacheControl;
                 delete (img: any).expires;
 
+                const t = performance.now();
                 const rawImageData = browser.getImageData(img);
+                const t1 = performance.now();
+                console.log("browser.getImageData: " + (t1 - t) + "ms");
                 const params = {
                     uid: tile.uid,
                     coord: tile.tileID,
diff --git a/src/source/raster_dem_tile_worker_source.js b/src/source/raster_dem_tile_worker_source.js
index 41e10139b..7c52e677a 100644
--- a/src/source/raster_dem_tile_worker_source.js
+++ b/src/source/raster_dem_tile_worker_source.js
@@ -19,7 +19,10 @@ class RasterDEMTileWorkerSource {
 
     loadTile(params: WorkerDEMTileParameters, callback: WorkerDEMTileCallback) {
         const {uid, encoding, rawImageData} = params;
+        const t = performance.now();
         const dem = new DEMData(uid, rawImageData, encoding);
+        const t1 = performance.now();
+        console.log("new DEMData: " + (t1 - t) + "ms");
 
         this.loaded = this.loaded || {};
         this.loaded[uid] = dem;