Switch from FreeImage to PistonDevelopers/image

[phdoerfler]

Or at least offer it as an alternative backend.

[chyh1990]

PristonDevelopers/image reimplements decoders for several image formats in Rust. But it seems impossible to support every variants of these formats (e.g. non-baseline jpeg, some PNG features).

So I use FreeImage to decode images. Maybe it would be good to offer a PristonDevelopers/image IO backend.

[theotherphil]

Note that PistonDevelopers contains an image processing library (which confusingly has the same name as this one - my fault for not checking for name collisions) which does use PistonDevelopers/image as its backend: https://github.com/PistonDevelopers/imageproc.

(I hope advertising other projects here isn't considered bad form.)