Ensure saturation/lightness between 0 and 1

[adamrutter]

The current Color:desaturate_by() function allows you to multiply the saturation value of the color to greater than 1, and less than 0. The result is obviously invalid colors being returned in such cases. See:

local color = Color.new("#ff0000")
local saturated_color = color:desaturate_by(1.1)
print(tostring(saturated_color)) -- result: #10cfffffffffffffff3fffffffffffffff3

The same goes for Color:lighten_by():

local color = Color.new("#ffffff")
local lighter_color = color:lighten_by(1.1)
print(tostring(lighter_color)) -- result: #119119119

I've added a couple of checks to each function so now:

print(tostring(saturated_color)) -- result: #ff0000
print(tostring(lighter_color)) -- result: #ffffff