Add opacity option to rgb2rgba

[kizu]

Right now rgb2rgba converts things to the color with the lowest alpha possible. However, there could be needs to have colours with bigger alpha (and more close to the target color). So, we could add opacity argument, that could be defined either in percents or in 0-1. This would as a multiplier to the existing alpha, like that: a = a + (1 - a) * opacity. This way we could even make it less opaque than possiible, but with color degradation.