void insert(LetterQuad ins) {
        LetterQuad n = next;
        next = ins;
        ins.next = n;
        ins.previous = this;
        n.previous = ins;
    }
create letter and append to previous LetterQuad

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