Flatten Transparency composited each region over white, so a blend mode
on the bare page read that white. Raster renders now draw their
background under the finished art, and the flattened color no longer
matched them: a Color Burn object on the bare page flattened to white
and rendered in its own color. The region now composites the art alone
and puts the white under it.
- Tests that relied on the white under the art now draw art under the
style (`every_builtin_style_paints_visibly_on_a_rectangle`) or compare
only whole-pixel pans with a fresh render
(`a_coarser_glow_kept_whole_draws_after_a_pan_as_made_again`).
- New tests cover every blend mode on white and blue backgrounds, a
thumbnail and the ink planes with a Screen glow, and
`over_background` at background and pixel alpha 0, 128 and 255.
dotCooCoo
5eb6b62632