Updated encoder to handle intra-only frames

Updated the encoder to handle frames that are coded
intra-only. Intra-only frames must be non-showable,
that is, the "show frame" flag must be set to 0 in
the frame header.

Tested by forcing the ARF frames to be coded intra-
only.

Note: The rate control code will need to be modified
to account for intra-only frames better than they
are currently handled.

Change-Id: I6a9dd5337deddcecc599d3a44a7431909ed21079