Found the cause of strange scaling where font scaling result is
unexpected - for instance too small/large a font rather than the
expected font size. This is caused by integer over- and underflow for
Fraction instances. Thus, a likely solution, is to swap the type of
numerator and denominator from ints to BigIntegers.

Signed-off-by: Jakob Brandsborg Ehmsen <jakobehmsen.github@gmail.com>