light bg formula is wrong

[sSolsta]

hi so this is something unrelated to my pull request that i found, light bg does NOT use the hsv v value to interpolate but it actually uses the sum of the r, g and b. i did find this in 1.9's code but 2.1 seems to behave identically from my testing.
almost-python pseudocode below for how it actually works, assuming rgb values are 0-255 and hsv's sv values are 0-100:

def lightBG(background: RGB, player1: RGB):
    hsv = background.toHSV()
    hsv.s -= 20 # pretend this is clamped between 0 and 100
    hsv.v += 20 # this too
    lightBG = hsv.toRGB()
    f = (background.r + background.g + background.b) / 150
    if f < 1:
        lightBG.r = lightBG.r * f + player1.r * (1 - f)
        lightBG.g = lightBG.g * f + player1.g * (1 - f)
        lightBG.b = lightBG.b * f + player1.b * (1 - f)
    return rgb

i found the hsv offsets in 1.9's PlayLayer::getLightBGColor and the actual interpolation was done in PlayLayer::updateVisibility, no idea if they're in the same functions in 2.1 though