Mouseup was turning into a click on whatever was the last thing clicked.
In this walkthrough, this would sometimes be the "Add Area" mode button,
which would re-enter the add area mode and throw away the user's drawing.
(I have no idea why.)
(closes#3986)
Root cause was the vertex shadow strokes were too big, esp on endpoints.
For normal lines 16px stroke width centers on the line, but for vertices
it extends beyond the vertex radius, stealing clicks.
Also switched some "globals" to module variables in draw.js
