Switch KisPainter to use floating point opacity instead of 0...255 integer range
Globally, we store opacity as a floating point value. It was quite weird that we converted the value to quint8, when passing to KisPainter, then back to float when passing to KoCompositeOp and then, finally, to channels_type.
Now KisPainter operates with floating-point opacity value. It removes all these extra conversions and errors when rounding.