Add utils to better quantize grayscale values to three bit indices while
compressing coverage masks.

Signed-off-by: Pavel Krajcevski <pavel@cs.unc.edu>

BUG=skia:

Review URL: https://codereview.chromium.org/669243003
diff --git a/AUTHORS b/AUTHORS
index ed4c657..51d2bf5 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -30,3 +30,4 @@
 Steve Singer <steve@ssinger.info>
 The Chromium Authors <*@chromium.org>
 Thiago Fransosi Farina <thiago.farina@gmail.com>
+Pavel Krajcevski <pavel@cs.unc.edu>