Created by: kovidgoyal
Rather than just passing the base font dir, pass the full font file path. This allows for user code that overrides fileURL() to properly map font file URLs as well.
I need this because in my application I am loading MathJax from IndexedDB, which means all mathjax files become blob urls, and my application has to map all MathJax URLs to Blob URLs, which it does by monkeypatching Ajax.fileURL() in the AuthorInit() function. See https://github.com/kovidgoyal/calibre/blob/master/src/pyj/read_book/mathjax.pyj#L15