Research tolerance when looking up font information for rects

[jerbob92]

When we are returning font information for rectangles returned by pdfium (added in #22), we have to lookup the first char of the rectangle to get the font information. The lookup of the first char has a tolerance to find the char (and we also need this tolerance to find it for every rectangle). Currently the tolerance is set to 5 (points), we have to research a little whether that is a good tolerance. Perhaps we could also start with lower and up the tolerance until we find something.