Fixed color lightening changing hue, as well as add screen color blending mode

[seanlnge]

The method used to lighten and darken colors was implemented in a way that changed the hue, so I rewrote the method to lighten or darken colors by a percentage between 0-1, instead of adding a single number n to red, green, and blue.

I also implemented the screen color blending mode using the method on wikipedia.