Pixen/Pixen — pull request #268 by interstateone (2012-12-02 21:00:07+00:00)
Title: Add shift lock while drawing with pencil tool

Mimics the behaviour in Photoshop, with the exception of where the lock point is set. PS will
set it to the mouse down point, this commit will set it to the point at which shift is first pressed. I think this behaviour is more intuitive, although it breaks with the convention.

Addresses issue [#232](https://github.com/Pixen/Pixen/issues/232).