Running with Mesa 20.1.0-devel (git-c7617d8908) GL renderer:
Radeon RX 550 Series (POLARIS11, DRM 3.27.0, 4.19.0-2-amd64, LLVM 8.0.1)
I found output-tranform test (a future patch) to produce exactly this much more
difference between Pixman and GL rendererers.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>