Transformer: fix problems when making an invalid line of reflection

Summary:
This fixes invalid lines of reflection (both line handle points are
at the same coordinate) currently breaking the UI.
This brings the reflection handles in front of the reflection button
so that you can still drag them to make a normal line afterwards, and
makes it so that you can't drag them directly on top of each other to
make a completely invalid line in the first place

Test Plan:
try to make an invalid reflection line in test.html
if you can, this commit is broken D:

Auditors: alex