Elad Shahar authored Jan 19, 2020 and Ell committed Jan 22, 2020

Changed the reflection method to find the middle of the slice the
user is drawing on, and reflecting on that. This reflects the slice
in place, making it easier to rotate it to the intended location.
This fix preserves the order the dabs are drawn on the
slices, to be counterclockwise.

Fixed the brush transform, by setting the reflect output variable,
and calculating accordingly.