fix(ImageMapper): getBoundsForSlice half voxel larger
fix(ImageMapper): getBoundsForSlice half voxel larger

ImageMapper.getBoundsForSlice was half voxel small.
It now uses ImageData.getSpatialExtent.

Kitware/vtk-js,Kitware/vtk-js,Kitware/vtk-js,Kitware/vtk-js