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): https://github.com/LibrePDF/OpenPDF/blob/3ddd942e99317cd56e4c871d2956fdda85a8a7cb/openpdf-html/src/main/java/org/openpdf/css/parser/property/PrimitivePropertyBuilders.java#L1562
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: https://github.com/LibrePDF/OpenPDF/blob/3ddd942e99317cd56e4c871d2956fdda85a8a7cb/openpdf-html/src/main/java/org/openpdf/pdf/ITextFontResolver.java#L286
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