Make contrast behave more like CSS/Photoshop legacy constrast

[nrkn]

What's Changing and Why

The contrast function in the color package was modified to behave more like the CSS contrast filter or Photoshop's legacy contrast mode (eg negative values move towards mid gray, positive values push colors near mid to their dark/light extremes)

What else might be affected

Nothing else in Jimp uses the contrast function, so theoretically, nothing.

Tasks

• Add tests

It appears that there are not currently any tests for contrast.

Was going to add some, but could not get Jimp to clean/build/run tests etc on Windows after cloning and doing NPM install - someone may want to look into cross platform testing, whether all build/clean/test dependencies are correctly installed etc. - I did notice some *nix shell specific things like rf in there

However I have manually verified by eye that an image adjusted with this algorithm from -1 to 1 by increments of 0.1 looks correct compared to both Photoshop and CSS

Somebody on whose system Jimp does correctly build should do a sanity check before merging this branch though, just to be sure that I haven't broken anything, unlikely though it seems.

• Update Documentation

contrast does not change from the user's point of view, no changes to documentation needed

• Update jimp.d.ts

All changes are internal to the contrast function, not required

• Add SemVer Label

[hipstersmoothie]

Sorry for taking so long to get to this! I do not have a windows machine so I'll do my best to get it building on windows