
### Fixed

- `U8x4Rgba::source_over` alpha channel now uses the correct Porter-Duff SRC_OVER
  formula: `out_a = src_a + dst_a * (1 - src_a)`. Previously used `a*a` which
  produced values too low for partially-transparent sources (#31).

### Changed

- Replaced `source_over_agrees_with_f32_path` test with `source_over_expected_values`
  comparing against hand-computed standard values (the f32 path has the same bug).

