Before this fix, the time_utc_usec output from blending was always 0.
This means that you wouldn't get a valid vehicle_gps_position/time_utc_usec
With this commit, UTC timestamps are blended according to weights for
all GPSes with a nonzero UTC timestamp value.
It would be possible to simply use the first valid UTC timestamp instead
of blending, but since the system timestamps are blended, it seems
suitable to blend UTC timestamps as well.
Øyvind Taksdal Stubhaug