Fix decoder handling of intra-only frames

This patch fixes bug 633:
https://code.google.com/p/webm/issues/detail?id=633

The first decoded frame does not have to be a keyframe,
it could be an inter-frame that is coded intra-only.

This patch fixes the handling of intra-only frames.

A test vector has also been added that encodes 3
intra-only frames at the start of the clip. The
test vector was generated using the code in the
following patch:
https://gerrit.chromium.org/gerrit/#/c/70680/

Change-Id: Ib40b1dbf91aae2bc047e23c626eaef09d1860147