skip prefilter for Linear and Constant

[johnnychen94]

Now that Linear() is just equivalent to BSpline(Linear()):

using ImageTransformations, ImageCore, ImageShow, TestImages, Interpolations
img = testimage("cameraman");

@btime imrotate($img, 0.3; method=Linear()); # 3.557 ms (21 allocations: 403.88 KiB)
@btime imrotate($img, 0.3; method=BSpline(Linear()));
# master: 3.491 ms (23 allocations: 660.02 KiB)
# PR: 3.639 ms (21 allocations: 403.88 KiB)

cc: @andrew-saydjari

[codecov]

Codecov Report

Merging #125 (a733467) into master (eb94fc2) will decrease coverage by 0.23%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
- Coverage   94.09%   93.86%   -0.24%     
==========================================
  Files           8        8              
  Lines         322      326       +4     
==========================================
+ Hits          303      306       +3     
- Misses         19       20       +1     

Impacted Files	Coverage Δ
src/compat.jl	77.77% <66.66%> (-5.56%)	⬇️
src/interpolations.jl	72.00% <100.00%> (+1.16%)	⬆️

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