Correct SPLITMV clamping

Prior to this fix, the clamping state of the last subblock partition
dominated, whereas the correct behavior is to clamp if any partition
needs clamping. This bug was introduced by v0.9.6-232-g6b25501

See also:
  [1]: http://code.google.com/p/webm/issues/detail?id=371
  [2]: https://bugzilla.mozilla.org/show_bug.cgi?id=696390

Change-Id: I444db492b4c4f05f039c7da6f4216da8207dc138