This tonemap was added as a temporary option only and if it'll be
needed again, it'll be better to implement is as either a spline
in OCIO or as a film response curve (as some of such curves were
added as a presets for RGB curves in Mango production SVN).
Also revert changes made to IMB_buffer_byte_from_float since it's
not actually needed anymore and makes it's clearer changes against
trunk.
3b9c3604c6