OTS: Add GSUB table support.

- Added src/gsub.{cc,h} for supporting GSUB table.
- Many fonts don't arrange script and feature tags in alphabetical order and
it might not be a security issue so no longer treat it as an error.

BUG=27131
TEST=http://code.google.com/p/ots/wiki/HowToTestOts (Verified with ~4000 TrueType/OpenType fonts) 
TEST=Confirmed that chrome can render the W3C font tests[1] for TTF font linking of Complex scripts(Khmer and Devanagari) as expected by hand.

[1] http://www.w3.org/International/tests/tests-html-css/list-fonts


git-svn-id: http://ots.googlecode.com/svn/trunk@57 a4e77c2c-9104-11de-800e-5b313e0d2bf3
diff --git a/src/ots.h b/src/ots.h
index 4badc54..8ba30b3 100644
--- a/src/ots.h
+++ b/src/ots.h
@@ -165,6 +165,7 @@
   F(gdef, GDEF) \
   F(glyf, GLYF) \
   F(gpos, GPOS) \
+  F(gsub, GSUB) \
   F(hdmx, HDMX) \
   F(head, HEAD) \
   F(hhea, HHEA) \