feat(grid): remove viewportIndex and only rely on viewportId (#3591)

Co-authored-by: Bill Wallace <wayfarer3130@gmail.com>

extensions/cornerstone/src/state.ts

@@ -10,13 +10,13 @@ const state = {
  * @return void
  */
 const setEnabledElement = (
-  viewportIndex: number,
+  viewportId: string,
   element: HTMLElement,
   context?: string
 ): void => {
   const targetContext = context || state.DEFAULT_CONTEXT;
 
-  state.enabledElements[viewportIndex] = {
+  state.enabledElements[viewportId] = {
     element,
     context: targetContext,
   };
@@ -27,8 +27,8 @@ const setEnabledElement = (
  *
  * @return {HTMLElement} Active viewport element.
  */
-const getEnabledElement = viewportIndex => {
-  return state.enabledElements[viewportIndex];
+const getEnabledElement = viewportId => {
+  return state.enabledElements[viewportId];
 };
 
 const reset = () => {