Add benchmarks

BenchmarkBilinearUp    100 24061376 ns/op 129.27 MB/s
BenchmarkBilinearDown  100 18101036 ns/op  25.46 MB/s
BenchmarkBicubicUp      50 34201956 ns/op  90.94 MB/s
BenchmarkBicubicDown   100 27431569 ns/op  16.80 MB/s
BenchmarkLanczosUp      50 43702498 ns/op  71.17 MB/s
BenchmarkLanczosDown    50 36882110 ns/op  12.49 MB/s