boolean isInvalid(StringBlock block, float gap) {
        if (isHead() || isTail())
            return false;
        if (x0 == Integer.MIN_VALUE || y0 == Integer.MIN_VALUE) {
            return true;
        }
        Rectangle bound = block.getTextBox();
        if (bound == null) {
            return false;
        }
        if (isRightToLeft()) {
            return x0 <0 && x0<bound.x;
            // ToDo check for ellipsis, not sure if it is on both sides of a character and need to be deducted as well
        } else {
            return x0 > 0 && bound.x+bound.width-gap < getX1();
        }
    }
create letter and append to previous LetterQuad

@param c
@param prev previous character
jme3-core/src/main/java/com/jme3/font/LetterQuad.java