The document explains above the tiger image (like, directly above it) that it is a test image meant to evaluate a hypothesis about fragment shader scheduling:

> Update (7 May): I did a test to see which threads in the fragment shader get scheduled to the same SIMD group, and there’s not enough coherence to make this workable. In the image below, all pixels are replaced by their mean in the SIMD group (active thread mask + simd_sum)

I cloned the piet-gpu repository and was able to render a very nice image of the Ghostscript tiger: https://imgur.com/a/swyW0gl