Only fire IRenderService.onRender when buffer changes occurred

Fixes #2836

src/browser/services/Services.ts

@@ -43,6 +43,10 @@ export interface IRenderService extends IDisposable {
   serviceBrand: any;
 
   onDimensionsChange: IEvent<IRenderDimensions>;
+  /**
+   * Fires when buffer changes are rendered. This does not fire when only cursor
+   * or selections are rendered.
+   */
   onRender: IEvent<{ start: number, end: number }>;
   onRefreshRequest: IEvent<{ start: number, end: number }>;