a_coarser_glow_kept_whole_draws_after_a_pan_as_made_again compared only
whole-pixel pans with a fresh render. At a fractional pan it now compares
the pixels right of the rectangle, where only the glow draws, with a fresh
render at the whole pixel nearest the pan (either neighbor on a half pixel).
RenderOptions::background and render_node_thumbnail's background are
straight-alpha colors, as over_background reads them; their docs said
premultiplied.