Add halving optimizations

[lilith]

Render1D should expand into a HalveHorizontal and Render1D when there is a huge downsampling factor.

HalveHorizontal and HalveVerticle can be shifted left until they meet, then combined into a Halve2D. Halve2D can be shifted left to the decoder (if the decoder supports built-in scaling).

The job will have to represent some optimization rules, such as permitting perceptual scaling instead of linear scaling for the first 1/3rd of the job, or halving prior to colorspace application.

[lilith]

Given that we are using downscaling in the jpeg decoder, the main hot path, this would only seem of value for large PNG images. Closing as wontfix for now.