Support max-height for img tag

[achuinard]

I don't believe the library is supporting a CSS max-height property. I am new to the library but found the same problem in Flying Saucer when I was using that. I came up with this to fix it:

    private static final class MaxWidthHeightSupportingRenderer extends ITextReplacedElementFactory {
        MaxWidthHeightSupportingRenderer(ITextOutputDevice outputDevice) {
            super(outputDevice);
        }

        @Override
        public ReplacedElement createReplacedElement(LayoutContext c, BlockBox box, UserAgentCallback uac, int cssWidth, int cssHeight) {
            Element e = box.getElement();
            if (e != null && "img".equals(e.getNodeName())) {
                String srcAttr = e.getAttribute("src");

                if (srcAttr != null && srcAttr.length() > 0) {
                    FSImage fsImage = uac.getImageResource(srcAttr).getImage();
                    if (fsImage != null) {
                        if (cssWidth != -1 || cssHeight != -1) {
                            long maxWidth = box.getStyle().asLength(c, CSSName.MAX_WIDTH).value();
                            long maxHeight = box.getStyle().asLength(c, CSSName.MAX_HEIGHT).value();
                            if (cssHeight > maxHeight && cssHeight >= cssWidth) {
                                fsImage.scale(-1, (int) maxHeight);
                            } else if (cssWidth > maxWidth) {
                                fsImage.scale((int) maxWidth, -1);
                            } else {
                                fsImage.scale(cssWidth, cssHeight);
                            }
                        }
                        return new ITextImageElement(fsImage);
                    }
                }
            }
            return super.createReplacedElement(c, box, uac, cssWidth, cssHeight);
        }
    }

I don't know how relevant that is to this project, as I just started using the library and need to learn more about how it works. Excited to see this though!

[achuinard]

I've created pull request #48 which I believe fixes this.