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.

Corrected image region parameter in CalculateMeanShiftVector

Changed pixel neighborhood array to image region iterator.
Updated image border handling.
Used variable length vectors to represent joint pixel.

ENH: removed test apTvRsRasterizationLinesStreaming, new GDAL streamed rasterization should be tested instead