imfilt_gaussian performance: when no NaNs, normalize using separability
imfilt_gaussian performance: when no NaNs, normalize using separability

When applicable, this provides an approximately 2x speed improvement.

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