./tests/cases/draw_polygon_mode.amber fails with SwiftShader

For example, the CI ubuntu clang debug build for #1014,
details at. https://source.cloud.google.com/results/invocations/2737c199-56c5-45e6-aff9-4a0d2a3829f5/targets

```
+ python3 ./tests/run_tests.py --build-dir /tmpfs/src/github/amber/build --use-swiftshader

/tests/cases/draw_polygon_mode.amber: Line 90: Probe failed at: 134, 82
Expected: 0.000000, 0.000000, 0.000000, 255.000000
Actual: 255.000000, 255.000000, 255.000000, 255.000000
Probe failed in 25 pixels

Command '['/tmpfs/src/github/amber/build/amber', '-q', './tests/cases/draw_polygon_mode.amber']' returned non-zero exit status 1.
```