Fix LoG kernel
Fix LoG kernel

Also increases kernel support, so that sum(kernel) ~ 0.  Note that this
kernel has a positive excitatory region instead of a negative one, so
filtered images will be inverted compared to the old version.

rened/Images.jl,cdsousa/Images.jl,twadleigh/Images.jl,JuliaPackageMirrors/Images.jl,rened/Images.jl,c42f/Images.jl,cdsousa/Images.jl,zhmz90/Images.jl,twadleigh/Images.jl,benmoran/Images.jl,zhmz90/Images.jl,timholy/Images.jl,c42f/Images.jl,angi2/Images.jl,angi2/Images.jl,benmoran/Images.jl