ENH: Mean shift. Changing internal image calculation in BeforeThreadedGenerateData to a functor filter
for multithreading support.

ENH: Adding functor filter for functor with (pixel,index) parameters and with a GetOutputSize() method
to specify the number of components per pixel.

ENH: Mean Shift. Extending input region for each thread to the RequestedRegion -> avoids block artefacts
at thread output regions boundaries.

ENH: Mean shift. VariableSizeVector's element-wise operators caused performance hit -> Changed to loops
over the vector dimension.