Reduce unnecessary allocations in `filtfft`

[BioTurboNick]

Several intermediate steps in filtfft are allocating when they don't need to be.

Results of this change on one of my operations:

Before: 86.106 ms (200 allocations: 140.55 MiB) After: 72.876 ms (196 allocations: 113.27 MiB)

19% improvement in time, 24% improvement in allocation size.

[codecov[bot]]

Codecov Report

Merging #231 (0c537f0) into master (6b51789) will increase coverage by 0.23%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #231      +/-   ##
==========================================
+ Coverage   91.60%   91.84%   +0.23%     
==========================================
  Files          11       11              
  Lines        1513     1594      +81     
==========================================
+ Hits         1386     1464      +78     
- Misses        127      130       +3     

Impacted Files	Coverage Δ
src/imfilter.jl	92.10% <100.00%> (+0.32%)	:arrow_up:
src/border.jl	92.93% <0.00%> (-0.17%)	:arrow_down:
src/extrema.jl	100.00% <0.00%> (ø)
src/kernel.jl	99.23% <0.00%> (+0.02%)	:arrow_up:
src/utils.jl	88.05% <0.00%> (+0.18%)	:arrow_up:
src/kernelfactors.jl	91.25% <0.00%> (+0.27%)	:arrow_up:
src/mapwindow.jl	86.90% <0.00%> (+0.46%)	:arrow_up:
src/borderarray.jl	93.33% <0.00%> (+0.74%)	:arrow_up:

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[timholy]

Thanks @BioTurboNick!