register external font defined in EPUB style sheet
with URL prefix "res://" and "file://" like this:
@font-face {
	font-family:"zw";
	src:
	url(../Fonts/zw.ttf),
	url(res:///opt/sony/ebook/FONT/zw.ttf),
	url(res:///mnt/us/fonts/zw.ttf),
	url(res:///mnt/sdcard/fonts/zw.ttf);
}