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[bot]]

Codecov Report

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

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

```diff @@ Coverage Diff @@ ## master #16 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 2 2 Lines 110 110 ========================================= Hits 110 110 ``` | [Impacted Files](https://codecov.io/gh/JuliaImages/ImageSmooth.jl/pull/16?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaImages) | Coverage Δ | | |---|---|---| | [src/algorithms/l0\_smooth.jl](https://codecov.io/gh/JuliaImages/ImageSmooth.jl/pull/16/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaImages#diff-c3JjL2FsZ29yaXRobXMvbDBfc21vb3RoLmps) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaImages). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaImages)

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