feat: 🎸 Synced Window Leveling

Syncs window leveling for vtkjs viewports in MPR 2D mode.

Closes: #558

extensions/vtk/src/utils/vtkViewportSubscriptionManager.js

@@ -0,0 +1,28 @@
+const subscriptions = [];
+
+// This is pretty hacky right now, but it makes sure we don't keep adding subscriptions.
+// TODO -> Nuke this and move it up a layer once we have more vigorous layout support.
+
+const vtkViewportSubscriptionManager = {
+  subscriptions,
+  pushSubscription(viewportIndex, subscription) {
+    if (!Array.isArray(subscriptions[viewportIndex])) {
+      subscriptions[viewportIndex] = [];
+    }
+
+    subscriptions[viewportIndex].push(subscription);
+  },
+  unsubscribe(viewportIndex) {
+    if (!subscriptions[viewportIndex]) {
+      return;
+    }
+
+    while (subscriptions[viewportIndex].length) {
+      subscriptions[viewportIndex].pop().unsubscribe();
+    }
+
+    subscriptions[viewportIndex] = null;
+  },
+};
+
+export default vtkViewportSubscriptionManager;