Add more synthetic scaler benchmarks
Micro optimize v8scaleN performance

Switching two operands shouldn't change performance, yet it does
and in a very consistent way. I don't understand this.

BenchmarkVerticalScalerN         417623       344219  -17.58%
Split benchmarks into groups so it's easier to select
Add naive rgb resizing

It reuses the current scaler which is not optimal as it increases
the number of taps by 4 on RGBA when resizing width.