The fix contains two parts:
1. Grease Pencil v3 now stores stroke `init_times` as a float attribute,
it's not enough precisiton for linux timestamp. Now the time value is
truncated to allow better precision on lower bits.
2. The previous logic for calculating stroke gap time of the build
modifier isn't correct, it used to only count two starting times as
gap time, now it's fixed with correct delta time so the gap time is
`init_time - (previous_init_time + previous_stroke_delta)`.