openpdf-html: Improve support for CSS `@font-face`

[alexdupre]

The current support for the @font-face rule has a few issues:

1. the src property is not correctly parsed, because the code expects a single url function, but there can be others (like format):

   OpenPDF/openpdf-html/src/main/java/org/openpdf/css/parser/property/PrimitivePropertyBuilders.java

   Line 1562 in 3ddd942

   public static class Src extends GenericURIWithNone {

2. the font file format is determined by the file extension, but the font url might not have a standard extension: the format function should be the preferred method to determine the type, with fallback on file extension:

   OpenPDF/openpdf-html/src/main/java/org/openpdf/pdf/ITextFontResolver.java

   Line 286 in 3ddd942

   return lower.endsWith(OTF) || lower.endsWith(TTF) || lower.contains(TTC_COMMA);

3. there is no way to embed fonts if the @font-face rule comes from an external @import (so no way to add -fs-pdf-font-embed: embed)
4. given that there is no support for the format function, probably it doesn't currently support the presence of multiple url with different formats, from which to choose the supported one

[andreasrosdal]

Pull requests welcome