scale_colors per-channel
scale_colors per-channel multipliers are:
(dmax is max(pre_mul[0...3], pre_mul is WB coeffs).
<p>
Final scaling is performed in copy_mem_image unless no_auto_bright is set:
<ul>
<li>histogram is analyzed (from right to left) to get level # where 1% of pixels are saturated
<li>output curve will map 0...level# to full output range
</ul>[/geshifilter-c]
-- Alex Tutubalin @LibRaw LLC
