This change improves efficiency and means that tile groups
cause negligible loss (+0.03% BDR) in the current default
encoding configurations (1 tile group no matter how many tiles)
with frame-based wrapping.

In future, when wrapper formats support tile groups, lengths
can be omitted from the last tile in every tile group.

Change-Id: Ia701e6720208fbfa727f9aef0b692a6aafbcc93f