fix(volumemapper): make vtkVolumeMapper inherit from vtkAbstractMapper3D

vtkVolumeMapper used to inherit from vtkAbstractMapper. vtkAbstractMapper3D defines methods that
relies on the implementation of getBounds, which is defined by vtkVolumeMapper. Moreover, the VTK
C++ implementation for vtkVolumeMapper also inherits from vtkAbstractMapper3D. This is not a
breaking change since vtkAbstractMapper3D also inherits from vtkAbstractMapper.

Sources/Rendering/Core/VolumeMapper/index.js

@@ -1,7 +1,7 @@
 import macro from 'vtk.js/Sources/macros';
 import * as vtkMath from 'vtk.js/Sources/Common/Core/Math';
 import Constants from 'vtk.js/Sources/Rendering/Core/VolumeMapper/Constants';
-import vtkAbstractMapper from 'vtk.js/Sources/Rendering/Core/AbstractMapper';
+import vtkAbstractMapper3D from 'vtk.js/Sources/Rendering/Core/AbstractMapper3D';
 import vtkPiecewiseFunction from 'vtk.js/Sources/Common/DataModel/PiecewiseFunction';
 
 const { BlendMode, FilterMode } = Constants;
@@ -170,7 +170,7 @@ const DEFAULT_VALUES = {
 export function extend(publicAPI, model, initialValues = {}) {
   Object.assign(model, DEFAULT_VALUES, initialValues);
 
-  vtkAbstractMapper.extend(publicAPI, model, initialValues);
+  vtkAbstractMapper3D.extend(publicAPI, model, initialValues);
 
   macro.setGet(publicAPI, model, [
     'sampleDistance',