public void merge(BitmapCharacterSet styleSet) {
        if (this.renderedSize != styleSet.renderedSize) {
            throw new RuntimeException("Only support same font size");
        }
        for (Entry<IntMap<BitmapCharacter>> entry : styleSet.characters) {
            int style = entry.getKey();
            if (style == 0) {
                throw new RuntimeException("Style must be set first. use setStyle(int)");
            }
            IntMap<BitmapCharacter> charset = entry.getValue();
            this.lineHeight = Math.max(this.lineHeight, styleSet.lineHeight);
            IntMap<BitmapCharacter> old = this.characters.put(style, charset);
            if (old != null) {
                throw new RuntimeException("Can't override old style");
            }

            for (Entry<BitmapCharacter> charEntry : charset) {
                BitmapCharacter ch = charEntry.getValue();
                ch.setPage(ch.getPage() + this.pageSize);
            }
        }
        this.pageSize += styleSet.pageSize;
    }
Merge two fonts.
If two font have the same style, merge will fail.
@param styleSet Style must be assigned to this.
author: Yonghoon
jme3-core/src/main/java/com/jme3/font/BitmapCharacterSet.java