Fix float conversion in vtkOpenGLGPUVolumeRayCastMapper.cxx
To reproduce, build with UBSan and run:
ctest -VV -R vtkRenderingVolumeCxx-TestGPURayCastMapperSampleDistance
To reproduce, build with UBSan and run:
ctest -VV -R vtkRenderingVolumeCxx-TestGPURayCastMapperSampleDistance