fix mask error for BW text, where I forgot to look at just the low-8 bits

fix mask error for BW text, where I forgot to look at just the low-8 bits
of the right mask when computing the number of "full" runs.

possibly related to crbug.com/178796
Committed: https://code.google.com/p/skia/source/detail?r=8408

[reed1, 2013-03-27 15:08:29]

Committed patchset #1 manually as r8408 (presubmit successful).