[Dev-luatex] Allow skippig the font cache in font.getfont

Hello, could font.getfont get an option to always return a new font table instead of returning the current one? While for many fonts a cached table makes sense, sometimes it would be useful to also have access to the ooriginal table, e.g. to see which characters are `used`, inspect the result of `font.addcharacters` or inspect other changes which might be applied to the underlying font from TeX. I attached a patch implementing this by adding a `nocache` parameter to `font.getfont`, such that `font.getfont(true, font_id)` always generates a new table. IMO `font.getfont(font_id, true)` would look nicer, but that wouldn't be compatible to older LuaTeX versions which always expect the font is as last parameter. Best regards, Marcel