During testing I accidentally managed to submit `NaN` as a pan
coordinate. This had the unfortunate side effect of bricking the
editor.
Given the serverity of an accidental `NaN` and that `NaN`s are not
impossible considering the amount of math involved in mouse move
operations, this commit introduces a simple validation step.
The new validation step should additionally be able to unstuck
anyone who have happened into this state by accident already.
