Draft: Use libpixman to dither gradients

In order to give pixman "more color" to work with, I switched to gradient builder to use floating-point color components rather than 8-bit integer components.

The gradients here look better, but there is still some light banding and distortion. I'm also not exactly sure how to set the dither offset optimally. So there might be room for improvement here.

Closes #149 (closed).