add GrHairLinePathRenderer

GrHairLinePathRenderer is for AA with msaa render target or non-AA with
non-msaa render target.  Comparing to GrDefaultPathRender, the
GrHairLinePathRenderer is able to batch multiple drawPath into one
single glDrawElements() call
Correct onGetStencilSupprt in GrShapePathRenderer
use bigger tolerance when determine whether it is a hairline

fast_len() is a imprecise computation of distance.  Use relaxed tolerance
use GrHairLinePathRenderer to stroke rect if width <= 1.0

GrHairLinePathRenderer is optimized to stroke path with width <= 1.0