Add subpixel dithering to curve lookups.

This will add subpixel dithering when applying curve upscaling on DNG/ARW/NEF images that have had a curve compression applied.

Basicly, if a curve is specified as:
```
0 -> 0
1 -> 3
2 -> 6
3 -> 10
4 -> 15
```
If an input value of 2 is encounted, it will look at the surrounding values and select one within half of the distance between them:
```
d = 10 - 3
out = 6 + random(0...1) * (d / 2) - (d / 4)
```
This means that out will be between 4.25 and 7.75