ENH: Improve Color.render performance for the case when contrast is 1

[lkeegan]

This makes draw() faster, e.g. here are snakeviz profiles for 100x Circle.draw() calls:

Without this PR:

With this PR:

Code used for above profiles:

from psychopy.visual.window import Window
from psychopy.visual.circle import Circle
import cProfile

window = Window(fullscr=True, units="height")
circle = Circle(window, radius=0.1, lineColor="black", fillColor="red")

with cProfile.Profile() as pr:
    for i in range(100):
        circle.draw()
        window.flip()
pr.dump_stats("profile.prof")

[TEParsons]

Good shout, I can see why this would speed things up!