Sets the point of origin for the rainbow effect if the RainbowType is set to RainbowEffect.POINT_OF_ORIGIN or RainbowType is set to RainbowEffect.LINE_OF_ORIGIN.
Note point of origin or line of origin may be set outside of the image to stretch the rainbow further if desired.
If this value is set to nil and RainbowType is set to RainbowEffect.POINT_OF_ORIGIN or RainbowType is set to RainbowEffect.LINE_OF_ORIGIN, then point of origin will automatically be set to the bottom right of the image.