... to make multi-color hard-edge gradient fills possible

Add a new "step" gradient-segment blending function, which is 0
before the midpoint, and 1 at, and after, the midpoint.  This
creates a hard-edge transition between the two adjacent color stops
at the midpoint.  Creating such a transition was already possible,
but required duplicating the same color at the opposing ends of two
adjacent stops, which is cumbersome.

(cherry picked from commit 68bf99e8)