Graphite - Transform Cage

I have implemented the transformation cage for Graphite, a work in progress vector editor written in the Rust programming language with Web Assembly. The transformation cage expanded on TrueDoctor's work adding a bounding box for selected layers whilst in select mode to allow the edges of the box to be draggable to scale the selection; and to add some blue squares on the corners and the edges to provide visual hints.  I also added the ability to rotate the selection by dragging 40px from the edges, as requested by Keavon.