Regenerate datashader visual references for alpha fix

Both tests use alpha < 1.0 (0.6 and 0.8). Pre-fix, alpha was ignored
and points rendered fully opaque; the references baked that in. The
fix correctly scales alpha, so the layered yellow-on-blue plot now
shows the expected color blend, and the continuous-color plot shows
the expected partial transparency. Reference images taken from CI
(visual_test_results_hatch-test.py3.14-stable, run 25553060603).

Author: timtreis