gputest gimark has unwanted black liquorice flakes

gputest's gimark subtest is misrendering—there are black speckles on the blue donuts:

Reproducible every time on Kabylake GT2. It doesn't happen with i965.

Originally reported by @eero-t via @tpalli.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information