EFX precision when controlling RGB channels

[mcallegari]

When an EFX controls RGB channels, it looks up for colors in a 256x256 Gradient table created here
https://github.com/mcallegari/qlcplus/blob/master/engine/src/gradient.cpp#L65

Vertically the pixmap has enough precision, but when a EFX moves horizontally, the color transitions are snappy.
Unfortunately ther's not much we can do, as EFX values themselves range from 0 to 255.
We could though lookup for the color a step before, when we're still in float and have more precision.
However this would mean to increase the Gradient size and I guess every hardcoded size will not work

Best thing would be to have a color lookup calculated mathematically, instead of a static bitmap lookup.

[janosvitok]

In QT, QColor::fromHslF() http://doc.qt.io/qt-4.8/qcolor.html#fromHslF may be used. For JS, here are the equations: https://en.wikipedia.org/wiki/HSL_and_HSV#From_HSL