Potential performance tweak on L0smooth

[johnnychen94]

Tweaking performance can be very difficult and there's no plan to investigate this issue. Running profiler gives me two suspicious performance hotspot

1. ImageFiltering.freqkernel: https://github.com/johnnychen94/ImageSmooth.jl/blob/1cb03e8949585e3431095e5869e706175df8e103/src/algorithms/l0_smooth.jl#L117-L118
2. fft! on complex number: https://github.com/johnnychen94/ImageSmooth.jl/blob/1cb03e8949585e3431095e5869e706175df8e103/src/algorithms/l0_smooth.jl#L159-L163