Fix temporary buffer planes

We were not splitting the buffer properly.
It's not a problem now, but will be with parallel resizes
Sort benchmarks in decreasing speed
Slightly change scaler interface and add plane copy
Add plane copy without any resize

This is especially useful for some resizes where we don't need to
resize the chroma planes
Make configuration fields public
Set proper plane sizes

We're not supposed to overwrite, but better let the compiler check
it for us
Extract resizePlane function and slightly change Resize signature
Pave the way for multi-threaded resizes
Resize planes in parallel

We match theorical speed-up of 1.5x on 420 content

benchmark                   old MB/s     new MB/s  speedup
BenchmarkBilinearUp           124.71       124.56    1.00x
BenchmarkBilinearUp-2         124.46       186.69    1.50x
BenchmarkBilinearUp-4         123.96       187.36    1.51x
BenchmarkBicubicUp             87.91        89.48    1.02x
BenchmarkBicubicUp-2           88.71       133.49    1.50x
BenchmarkBicubicUp-4           87.66       134.47    1.53x
BenchmarkLanczosUp             68.81        70.27    1.02x
BenchmarkLanczosUp-2           69.24       105.08    1.52x
BenchmarkLanczosUp-4           69.27       105.29    1.52x
BenchmarkBilinearDown          25.81        25.77    1.00x
BenchmarkBilinearDown-2        25.43        38.62    1.52x
BenchmarkBilinearDown-4        25.68        38.72    1.51x
BenchmarkBicubicDown           16.71        16.69    1.00x
BenchmarkBicubicDown-2         16.67        24.92    1.49x
BenchmarkBicubicDown-4         16.69        25.03    1.50x
BenchmarkLanczosDown           12.47        12.45    1.00x
BenchmarkLanczosDown-2         12.44        18.59    1.49x
BenchmarkLanczosDown-4         12.48        18.69    1.50x
Fix interlaced resize and add some benchmarks

Interlaced resize was creating invalid slices at the last iteration
which will panic, even if the slice is never accessed.
Instead, we maintain an index and never access outside the slice.
Furthermore, with interlaced resize, the last line is width pixels,
not pitch pixels, which will overread/overwrite on the second field.
Tighten slice boundaries
Resize interlaced fields in parallel
Split planes into slices and resize them in parallel

This is the biggest speed-up which should scale, in theory, up to
one line per slice. However there is a loss in efficiency which I
have not been able to determine yet, maybe some unoptimized
scheduler decisions

benchmark                   old MB/s     new MB/s  speedup
BenchmarkBilinearUp           124.71       125.56    1.01x
BenchmarkBilinearUp-2         124.46       216.89    1.74x
BenchmarkBilinearUp-4         123.96       431.02    3.48x
BenchmarkBicubicUp             87.91        92.40    1.05x
BenchmarkBicubicUp-2           88.71       162.08    1.83x
BenchmarkBicubicUp-4           87.66       319.32    3.64x
BenchmarkLanczosUp             68.81        72.03    1.05x
BenchmarkLanczosUp-2           69.24       127.21    1.84x
BenchmarkLanczosUp-4           69.27       248.22    3.58x
BenchmarkBilinearDown          25.81        24.97    0.97x
BenchmarkBilinearDown-2        25.43        47.63    1.87x
BenchmarkBilinearDown-4        25.68        83.75    3.26x
BenchmarkBicubicDown           16.71        16.50    0.99x
BenchmarkBicubicDown-2         16.67        32.20    1.93x
BenchmarkBicubicDown-4         16.69        56.26    3.37x
BenchmarkLanczosDown           12.47        12.21    0.98x
BenchmarkLanczosDown-2         12.44        23.97    1.93x
BenchmarkLanczosDown-4         12.48        42.35    3.39x
Rename Adapter to Converter which better reflects its purpose
Add Convert helper function

This is sub-optimal but convenient
Add image copy benchmarks

It's slower as we increase cpu count