fix: grab cornerstone instance when called, not module load (#1078)

Reduces the chance that cornerstone has not yet been registered (resulting in `undef`)

src/mixins/segmentation/renderBrushMixin.js

@@ -1,16 +1,15 @@
 import external from './../../externalModules.js';
 import store, { getModule } from './../../store/index.js';
 
-const { cornerstone } = external;
-const { getters, configuration } = getModule('segmentation');
-
 /**
  * Called by the event dispatcher to render the image.
  *
  * @param {Object} evt - The event.
  * @returns {void}
  */
 function renderBrush(evt) {
+  const { cornerstone } = external;
+  const { getters, configuration } = getModule('segmentation');
   const eventData = evt.detail;
   const viewport = eventData.viewport;