Fix decoder resolution change with tiles

There was a bug with the decoder that if you started the decoder
with more threads than the first frame had tile columns. Afterwards
tried to decode a frame with more tile columns than the first frame,
the decoder would hang. E.g. run vpxdec --threads=4. The first frame
had two tile columns, then the next key frame had 4 tile columns, the
decoder would hang. If you started with 4 tiles and switched to 2
tiles the decoder would be fine. The issue is that the worker the thread
loop is using is stale.

I added a test vector "vp90-2-14-resize-848x480-1280x720.webm" that
exhibited the bug.

Change-Id: I7bdd47241a52ac0fe1c693a609bc779257e94229