mpegts: Remove one 64-bit integer modulus operation per packet

The common case of the pointer having increased by one packet (which results
in no change to the modulus) can be detected with a 64-bit subtraction,
which is far cheaper than a division on many platforms.

           Before          After
           Mean   StdDev   Mean   StdDev  Change
Divisions   248.3  8.8      51.5   7.4    +381.7%
Overall    2773.2 25.6     2372.5 43.1     +16.9%

Signed-off-by: Martin Storsjö <martin@martin.st>