Optimize LoG generation Magneti Selection tool

For Colorize Mask we set kernel size to be (4 * radius + 1) to ensure
that the watershed algorithm stops not reaching the border of the
color blob. In the selection tool, it is the opposite. We should stop
right at the edge. Therefore, the kernel size should be (2 * radius + 1)
and should not include the wrapped art of the LoG surface.