Changed Material.overdraw from a boolean to a number

Find comments and demo here: https://github.com/mrdoob/three.js/pull/3407

The previous boolean type allowed for either a 0 or a 1 pixel overdraw
for fixing antialiasing gaps in CanvasRenderer. I've found the best
value to be 0.35, though this may be browser/gpu specific. Should be
backwards compatible.