Add missing buffer lengths
Sort benchmarks in decreasing speed
Add parallelism between planes

We're benchmarking yuv 420 so the theorical speed up is 1.5x

benchmark                   old MB/s     new MB/s  speedup
BenchmarkBilinearUp           125.31       124.96    1.00x
BenchmarkBilinearUp-2         125.21       187.36    1.50x
BenchmarkBilinearUp-4         125.51       186.91    1.49x
BenchmarkBicubicUp             90.36        88.86    0.98x
BenchmarkBicubicUp-2           90.36       133.03    1.47x
BenchmarkBicubicUp-4           90.52       133.60    1.48x
BenchmarkLanczosUp             70.88        70.14    0.99x
BenchmarkLanczosUp-2           70.72       105.08    1.49x
BenchmarkLanczosUp-4           70.78       105.25    1.49x
BenchmarkBilinearDown          25.11        25.74    1.03x
BenchmarkBilinearDown-2        24.97        38.75    1.55x
BenchmarkBilinearDown-4        25.01        38.46    1.54x
BenchmarkBicubicDown           16.59        16.59    1.00x
BenchmarkBicubicDown-2         16.62        24.73    1.49x
BenchmarkBicubicDown-4         16.62        24.75    1.49x
BenchmarkLanczosDown           12.45        12.25    0.98x
BenchmarkLanczosDown-2         12.43        18.24    1.47x
BenchmarkLanczosDown-4         12.46        18.35    1.47x
Split planes into slices and resize them in parallel

We don't match theorical speed-up, maybe due to some code
inefficiences and/or unoptimal job scheduling ?
It also should scale higher than 4, but I can't test it.

benchmark                   old MB/s     new MB/s  speedup
BenchmarkBilinearUp           125.31       125.31    1.00x
BenchmarkBilinearUp-2         125.21       234.03    1.87x
BenchmarkBilinearUp-4         125.51       428.29    3.41x
BenchmarkBicubicUp             90.36        88.91    0.98x
BenchmarkBicubicUp-2           90.36       156.92    1.74x
BenchmarkBicubicUp-4           90.52       305.52    3.38x
BenchmarkLanczosUp             70.88        70.02    0.99x
BenchmarkLanczosUp-2           70.72       127.73    1.81x
BenchmarkLanczosUp-4           70.78       245.87    3.47x
BenchmarkBilinearDown          25.11        25.73    1.02x
BenchmarkBilinearDown-2        24.97        48.12    1.93x
BenchmarkBilinearDown-4        25.01        84.30    3.37x
BenchmarkBicubicDown           16.59        16.57    1.00x
BenchmarkBicubicDown-2         16.62        32.31    1.94x
BenchmarkBicubicDown-4         16.62        55.41    3.33x
BenchmarkLanczosDown           12.45        12.25    0.98x
BenchmarkLanczosDown-2         12.43        23.47    1.89x
BenchmarkLanczosDown-4         12.46        41.18    3.30x