When computing the barycenter between two colors, the some of the two
factors a and (1-a) was equal to 255/256 instead of 1.
Change-Id: Ia9a779d43470ef42d9430ad730e842da0f007140
KDColor::blend used to produce different colors when blending two
identical colors (ex : use draw_string in Python to print white text on
a white background).
blend now escapes early when its two color arguments are identical.
Change-Id: I01dc5a0d5e4e6a20e09fee0f346dafc313dae97b
