The presentation time in fMP4 is calculated by adding and subtracting
3 values. All 3 values are currently converted to microseconds first
before the calculation, leading to rounding errors. The rounding errors
can be avoided by doing the conversion to microseconds as the last step.
For example:
In timescale 96000: 8008+8008-16016 = 0
Rounding to us first: 83416+83416-166833=-1
#minor-release
PiperOrigin-RevId: 406809844
d1e13b629d