ThumbRangeFor
virtual float ThumbRangeFor(int32 channel);
float range = ThumbRangeFor(channel);
bottomRight.Set(leftTop.x, leftTop.y + ThumbRangeFor(channel));
bottomRight.Set(area.left + ThumbRangeFor(channel), leftTop.y);
float range = ThumbRangeFor(channel);
rect.top -= ThumbRangeFor(0);
rect.right += ThumbRangeFor(0);
float range = ThumbRangeFor(fCurrentChannel);