Changed material.overdraw from a boolean to a number

The previous boolean type for material.overdraw 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. The change should be backwards compatible.

Demo (line 345 is where material created):

overdraw = 0 (default, previously false) shows diagonal lines
http://neilscratch.highermedia.us/three_js/canvas_numeric_overdraw.html

overdraw = 1 (previously true) is too much 
http://neilscratch.highermedia.us/three_js/canvas_numeric_overdraw.html#1

overdraw = 0.35 is just right
http://neilscratch.highermedia.us/three_js/canvas_numeric_overdraw.html#0.35

(Forgive sloppy code of above demo. Hastily extracted from another project)