This change adds a new compiler directive 'kage:unit' to Kage. This
takes one of these two values: 'pixel' and 'texel'. The default value
is 'texel'.
With the pixel-unit mode, all the built-in functions treats pixels
instead of texels, and the texCoord argument of Fragment is in pixels.
This simplifies shader programs as programs no longer have the notion
of texels.
With the texel-unit mode, the behavior is the same as the current
behavior.
Closes#1431
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
