Add CPU gaussian sorting

[Meakk]

Is your feature request related to a problem? Please describe.
When changing the camera orientation, it is required to sort the gaussians by depth in order to have a correct result.
Sorting is currently executed on the GPU using compute shaders.
However it's not supported on macOS and the GPU sorting can be slow on low-end GPUs.
Let's consider CPU sorting using vtkDepthSortPolyData

Gaussians visualization is done in vtkext\private\module\vtkF3DPointSplatMapper.cxx file, and sorting shader is executed in the method vtkF3DSplatMapperHelper::SortSplats

We probably need to rename the method to SortSplatsGPU and add a new SortSplatsCPU function. The CPU function could be enable on macOS only for now, but it would be useful to have a way to let the user select which backend to use.