[pxl-build] Support gamma, linearization, and color spaces

The [`image`](https://github.com/PistonDevelopers/image) crate loads image data from disk without gamma correction, linearization, or color-space correction.

We should respect gamma and color space information in image resource source PNGs and JPEGs and load images as linear RGB. For a good breakdown of these issues, see [this crate](https://github.com/Ogeon/palette).

Ideally the `image` crate could add this functionality, requested in [this issue](https://github.com/PistonDevelopers/image/issues/786). If not, we might consider using a library that allows loading image metadata, or switching to another image decoding library.