Log Message:
Update to 0.9.28:

Overview of changes leading to 0.9.28
Monday, April 28, 2014
=====================================

- Unbreak old-spec Indic shaping. (bug 76705)
- Fix shaping of U+17DD and U+0FC6.
- Add HB_NO_MERGE_CLUSTERS build option.  NOT to be enabled by default
  for shipping libraries.  It's an option for further experimentation
  right now.  When we are sure how to do it properly, we will add
  public run-time API for the functionality.
- Build fixes.