use fast-pt case when radius <= 1/2, instead of just hairlines

use fast-pt case when radius <= 1/2, instead of just hairlines
Committed: https://code.google.com/p/skia/source/detail?r=6616

[reed1, 2012-11-29 16:26:58]

also had to reorder the choose routine, since bw can go faster for more cases
(radius <= 1/2) than the aa-case (radius == 0)

[robertphillips, 2012-11-29 18:52:44]

LGTM