A frame fraction was taken of the render's short edge, and render_scale
folds the zoom into the region the render stands for. Zoomed to 1:1 on a
corner, clarity, texture and dehaze drew a quarter of the halo the export
gets, though the docs promise they preview honestly at any zoom.
RenderScale now carries the whole framed frame (within), and a frame
fraction is measured against it; at fit nothing changes.