DO NOT MERGE CTS Holo: Enlarge the threshold from 1 to 2

Optimized Skia implementation may have 1 bit difference with the accurate result,
which is same as Google's default Skia implementation.
But other vendor may choose different optimization with Google's,
so the maximum difference between Google's C optimization and other vendor's optimization may be 2

Change-Id: Iac19e6b8bd53b3932ebb38c12c4b795ee31316f8
Signed-off-by: Xu Ziyi <zyxu@marvell.com>