Add --enable-true-color-rendering flag
This feature has been split off from --enable-color-correct-rendering
enable-color-correct-rendering is a tweaking of the existing color
awareness to be more consistent and robust, and will only be noticeable
on non-sRGB monitor.
enable-true-color-rendering does all blending and interpolation
operations in a physically correct linear space, and has perceptually
noticeable differences on all monitors.