interlace: Increment phase_index before checking if we're at the end of the phase

Incrementing it afterwards will always have to phase_index >= 1 and we will never be at the beginning (0) of the phase again, and thus never reset timestamp tracking accordingly.

This was broken in bea13ef4 in 2010, and causes interlace to run into integer overflows after 2^31 frames or about 5 hours at 29.97fps. Due to usage of wrong types for the integers this then causes negative numbers to be used in calculations and all calculations spectacularly fail, leading to all following buffers to have the timestamp of the first buffer minus one nanosecond.