Previously the delta phase was fixed point fractional with 2^32 fractions,
which caused inaccuracies in the output frequency, unless the input
frequency*2^32 was divisable by the sample rate.
This patch improves frequency accuracy by tracking subfractions of the delta
phase fractions. For this we are using a denominator which is a multiple of the
sample rate, making sure that integer frequencies are always accurately
represented.
Signed-off-by: Marton Balint <cus@passwd.hu>