Transformer: Fix reflection line loading from old json

Summary:
Loading a reflection line from json generated before the
reflection line's UI change (where there is one point instead of 2),
could sometimes result in funny problems happening, because some
assumptions about the relationship between the points of reflection
were violated. This commit should fix these issues by normalizing
the reflection points when adding a reflection tool

Test plan:
Load
https://www.khanacademy.org/devadmin/content/items/x7d8d3066a3729df5
locally, and play with the reflection tool
Check
 * that it is always on a 45 degree angle
 * that it is always on a snap (not at a 0.25 unit alignment)

Auditors: alex