The SDR shoulder in ToneMap::display_hdr ran at neutral Contrast, Whites
and Blacks too, so an unedited JPEG or PNG exported neutral 255, 254 and
253 one level darker and a white background never came out as 255. The
shoulder now shapes edited curves only; neutral sliders are the identity
up to white and clip above it. Edited curves render as before (V1).
Tests: the identity test runs to 1.0, a new pipeline test renders the
issue's neutral greys back exactly, the sRGB ramp round trip is exact,
and a guard keeps the shoulder on edited curves.
Gert Pellin
c5cebdef39