Force show_frame=1 when show_existing_frame=1

When showing a previously decoded frame, we need to
explicitly set the show_frame flag.

For the current frame being decoded this flag is
explicitly set in the frame header.

This should fix WebM Issue 696:
http://code.google.com/p/webm/issues/detail?id=696

Change-Id: I5751a809813f88d2ca6f62c47c3878475ff9ba8d