Fixing star color intensity setting in OpenGL mode.

While it is implemented by having a full palette cached in QPainter
mode, it is implemented by converting from RGB to HSV, readjusting the
saturation, and converting back in the OpenGL mode. Looks better to
me.

Also adding a FIXME for later on randomly-stumbled-upon code in
StarObject.

BUG
CCMAIL: kstars-devel@kde.org, hdevalence@gmail.com