Factorize xwidth
Factorize some hgen code
Reduce dependencies a little bit

Not gonna improve much though

Benchmark                      old ns/op    new ns/op    delta
BenchmarkHorizontalScaler2          8475         8345   -1.53%
BenchmarkHorizontalScaler4          9995         9920   -0.75%
BenchmarkHorizontalScaler6         15100        14955   -0.96%
BenchmarkHorizontalScaler8         15110        14940   -1.13%
BenchmarkHorizontalScaler10        34861        34421   -1.26%
BenchmarkHorizontalScaler12        41402        40942   -1.11%
BenchmarkHorizontalScalerN         48582        47962   -1.28%