skip prefilter step for BSpline(Linear()) and BSpline(Constant())

[johnnychen94]

This difference is originally identified by @andrew-saydjari in #116 (comment) and I believe we could optimize it away:

img = testimage("cameraman")

imrotate(img, 0.3; method=Linear()) == imrotate(img, 0.3; method=BSpline(Linear())) # true

@btime imrotate($img, 0.3; method=Linear()); # 3.479 ms (21 allocations: 403.88 KiB)
@btime imrotate($img, 0.3; method=BSpline(Linear())); # 3.491 ms (23 allocations: 660.02 KiB)

[johnnychen94]

closed by #125