perf: pre-compute FFT plan

[johnnychen94]

using TestImages
using ImageSmooth

img = testimage("cameraman")

fₛ = L0Smooth() # using default arguements
imgₛ = smooth(img, fₛ)

This drops the runtime from ~280ms to ~240ms in my machine.

cc: @JKay0327

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (58354b6) compared to base (d1a9afb).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #16   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          110       110           
=========================================
  Hits           110       110           

Impacted Files	Coverage Δ
src/algorithms/l0_smooth.jl	100.00% <100.00%> (ø)

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