avutil/mathematics/av_add_stable: avoid unneeded variable

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/mathematics/av_add_stable: check for the common case of inc=1

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
av_add_stable: Add fast special case where step can be represented exactly

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/mathematics/av_add_stable: Avoid av_cmp_q() call

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>