Splash::gouraudTriangleShadedFill(): relax assertion threshold
On the reproducer file of https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=34505, the 1e-10 threshold is hit. Relaxing it to 1e-9 avoids the assertion.
clusterfuzz-testcase-minimized-gdal_fuzzer-5006856383758336.pdf