Remove one branch from inner loop

It doesn't bring much though

BenchmarkBilinearUp-4         662.60       662.88    1.00x
BenchmarkBicubicUp-4          415.47       440.23    1.06x
BenchmarkLanczosUp-4          332.47       332.64    1.00x
BenchmarkBilinearDown-4       112.82       118.09    1.05x
BenchmarkBicubicDown-4         70.45        74.17    1.05x
BenchmarkLanczosDown-4         43.72        43.59    1.00x
BenchmarkBicubicIUp-4         414.14       433.42    1.05x
BenchmarkBicubicIDown-4       413.59       433.78    1.05x
Tweak scaler loop and extract some performance

benchmark                   old MB/s     new MB/s  speedup
BenchmarkBilinearUp           195.49       225.05    1.15x
BenchmarkBilinearUp-2         369.17       399.77    1.08x
BenchmarkBilinearUp-4         660.62       779.90    1.18x
BenchmarkBicubicUp            127.94       140.48    1.10x
BenchmarkBicubicUp-2          228.36       253.48    1.11x
BenchmarkBicubicUp-4          440.92       484.61    1.10x
BenchmarkLanczosUp             96.17       104.23    1.08x
BenchmarkLanczosUp-2          178.03       190.81    1.07x
BenchmarkLanczosUp-4          333.71       366.12    1.10x
BenchmarkBilinearDown          34.46        34.54    1.00x
BenchmarkBilinearDown-2        67.31        66.78    0.99x
BenchmarkBilinearDown-4       116.95       116.12    0.99x
BenchmarkBicubicDown           21.50        22.31    1.04x
BenchmarkBicubicDown-2         42.00        43.39    1.03x
BenchmarkBicubicDown-4         74.03        76.29    1.03x
BenchmarkLanczosDown           12.67        12.39    0.98x
BenchmarkLanczosDown-2         24.44        23.65    0.97x
BenchmarkLanczosDown-4         43.51        43.31    1.00x
BenchmarkBicubicIUp           127.89       140.29    1.10x
BenchmarkBicubicIUp-2         229.37       248.82    1.08x
BenchmarkBicubicIUp-4         433.06       478.50    1.10x
BenchmarkBicubicIDown         128.31       140.29    1.09x
BenchmarkBicubicIDown-2       228.19       253.48    1.11x
BenchmarkBicubicIDown-4       434.51       475.13    1.09x