fix Stretch-both-sides scaling

When you turn on Stretch-both-sides, the drag handle no longer tracks cursor movement. Instead, the total width (or height or whatver) of the prim increases by the distance the cursor moves, rather than adjusting both edges by that amount. When you are done dragging the edge, the cursor is no longer on the drag handle. This is a CS101 mistake (really, I did exactly this in my first graphics editor I wrote in class), and can be solved by a simple multiply by 2 in the code.

Can we get this fixed soon? It's such a simple fix!