This replaces the built-in shaders with Kage shadres. This is a
refactoring and doesn't degrade performance:
```
go test -bench=^BenchmarkColorMScale$ -run=^$ . -count=5
```
```
name old time/op new time/op delta
ColorMScale-8 978ns ±15% 1184ns ±46% ~ (p=0.413 n=4+5)
```
A follow-up change to remove the built-in shaders is needed.
Closes#2364