NGPre tile source: factor path normalization

Also do the normalization now on the final path, rather than the base.
This allows for additional modifications that are useful for Google
Cloud storage, where the only way to get back proper CORS headers seems
to be to use their object get API.

django/applications/catmaid/static/js/tile-source.js

@@ -1001,8 +1001,7 @@
           'Your browser does not support features required for NeuroglancerPrecomputed mirrors');
       }
 
-      this.datasetURL = this.baseURL.substring(0, this.baseURL.lastIndexOf('/'))
-          .replace(/^gs:\/\//, 'https://storage.googleapis.com/');
+      this.datasetURL = this.baseURL.substring(0, this.baseURL.lastIndexOf('/'));
 
       let sliceDims = this.baseURL.substring(this.baseURL.lastIndexOf('/') + 1);
       this.sliceDims = sliceDims.split('_').map(d => parseInt(d, 10));
@@ -1065,8 +1064,27 @@
         });
     }
 
+    normalizeUrl(path) {
+      // If this path points to Google Cloud storage, rebuild the path so that it uses the object
+      // get method (see https://cloud.google.com/storage/docs/json_api/v1/objects/get).
+      // Turn gs://h01-release/data/20210601/4nm_raw/
+      // into https://www.googleapis.com/storage/v1/b/h01-release/o/data%2F20210601%2F4nm_raw%2F
+      // Note that the new URL will also get a ?alt=media appended.
+      if (path.startsWith("gs://")) {
+          let components = path.split("/");
+          if (components.length < 4) {
+            throw new CATMAID.ValueError(`Unsupported Google Cloud URL: ${path}`);
+          }
+          let bucketName = components[2];
+          let subPath = components.slice(3).join("%2F");
+          return `https://www.googleapis.com/storage/v1/b/${bucketName}/o/${subPath}?alt=media`;
+      }
+
+      return path;
+    }
+
     getCanaryUrl(project, stack) {
-      return `${this.rootURL}/info`;
+      return this.normalizeUrl(`${this.rootURL}/info`);
     }
 
     populateDatasetAttributes(zoomLevel = 0) {
@@ -1212,7 +1230,11 @@
         () => {
           let request = (options) => {
             if (noCache) {
-              url += "?nocache=" + Date.now();
+              if (url.indexOf('?') >= 0) {
+                url += "&nocache=" + Date.now();
+              } else {
+                url += "?nocache=" + Date.now();
+              }
             } else {
               if (this.canaryCache.has(url)) {
                 return this.canaryCache.get(url);