Stray Scanner quaternions

[rwcohn]

Not a bug, but a possible inconvenience to deal with for users expecting the quaternions to match transformations from ARKit's coordinate space.

The quaternions output by Stray Scanner can be reverted to ARKit's camera coordinate space by multiplying each quaternion q by the quaternion q' = qw=0, qi=1, qj=0, qk=0, i.e. quaternion_multiply(q, q'), which serves as an inversion of the x-axis rotation applied by Stray Scanner to ARKit's quaternions.