Don't dispose of texture atlas in canvas addon

This would cause problems if the atlas is shared between multiple
terminals.

addons/xterm-addon-canvas/src/BaseRenderLayer.ts

@@ -74,7 +74,6 @@ export abstract class BaseRenderLayer extends Disposable implements IRenderLayer
 
     this.register(toDisposable(() => {
       this._canvas.remove();
-      this._charAtlas?.dispose();
     }));
   }